TO  ALL  TEACHERS. 


SCKOOZ.   BOOKS. 

SMILEY'S   GEOGRAPHY   AND   ATLAS,  and  SACRED 
AND  ANCIENT  GEOGRAPHY  FOR  SCHOOLS. 

The  above  works  -will  be  foiind  useful  and  very  valuable  as  works  of  refer- 
ence, as  well  as  for  schools.  The  Maps,  composing  the  Atlases,  will  be  fouml 
equal  in  execution  and  correctness  to  those  on  the  most  extensive  scale.  The 
author  has  received  numerous  recommendations,  among  which  are  the  fol- 
lowirg : 

Dear  Sir — I  have  looked  over  your  "  Easy  Introduction  to  the  Study  o/" 
GeogYaphy^''^  together  with  your  "  Improved  Atlas.''''  I  have  no  hesitation  m 
declaring,  that  I  consider  them  works  of  peculiar  merit.  They  do  honour  to 
your  industry,  research,  and  talent,  and  I  am  satisfied,  will  facilitate  the  im- 
provement of  the  student  in  geographical  science. 

With  sentiments  of  sincere  consideration,  I  am  yours  truly, 

WM.  STAUGHTON,  D.  D. 
President  of  Columbia  College^  District  of  Columbia, 

Mr.  Thomas  Smiley. 

Philadelphia,  Sept.  1, 1823. 


Extract  from  the  Minutes  of  the  Philadelphia  Academy  of  Teaehen. 

J^ovember  1, 1823. 
Resolved  unanimously,  That  the  Academy  of  Teachers  highly  approve  the 
superior  merits  of  Mr.  Smiley's  "  Easy  Introduction  to  the  Study  of  Geogra- 
phy,'''' and  the  accompanying  Atlas,  and  cordially  recommend  them  to  the  pa- 
tronage of  the  public. 

B.  MAYO,  President. 
1. 1.  Hitchcock,  Secretary, 


THE  NEW  FEDERAL  CALCULATOR,  or  SCHOLAR'S 

ASSISTANT.  Containing  the  most  concise  and  accurate 
Rules  for  performiug  the  operations  in  common  Arithmetic 5 
together  with  numerous  Examples  under  each  of  the  Rules, 
varied  so  as  to  make  them  contormable  to  almost  every  kind 
of  business.     For  the  Use  of  Schools  and  Counting  Houses. 


By  Thomas  T.  Smilej,  Teacher:  author  of  An  Easy  Intro- 
duction to  the  Study  of  Geography.  Also,  of  Sacred  Geo- 
graphy for  the  Use  of  Schools. 

Among  the  numerous  recommendations  received  to  the  work,  are  the  following; 

Mr.  John  GricxG.  Phila.  March  8,  1825. 

Sir — I  have  examined  with  as  much  care  as  my  time  would  admit,  "  The 
New  Federal  Calculator,"  by  Thomas  T.  Smiley.  It  appears  to  me  to  be  a 
treatise  on  Arithmetic  of  considerable  merit.  There  are  parts  in  Mr.  Smiley 's 
work  which  are  very  valuable  ;  the  rules  given  by  him  in  Barter,  Loss  and 
Gdin,  and  Exchange,  are  a  great  desideratum  in  a  new  system  or  treatise  on 
Arithmetic,  and  renders  his  book  superior  to  any  on  the  subject  now  in  use  ; 
and  when  it  is  considered  that  the  calculations  in  the  work  are  made  in  Fede- 
ral Money,  the  only  currency  now  known  in  the  United  States,  and  that  ap- 
propriate questions  ioliow  the  difTprent  rules,  by  which  the  learner  can  be  ex- 
ercised as  to  his  understanding  of  each  part  as  he  progresses  ;  I  hesitate  not  to 
say,  that,  in  my  opinion,  it  is  eminently  calculated  to  promote  instruction  in 
the  science  on  which  it  treats.  Mr.  Smiley  deserves  the  thanks  of  the  public 
and  the  encouragement  of  teachers,  for  his  attempt  to  simplify  and  improve 
the  method  of  teaching  Arithmetic.     I  am  yours  respectfully, 

WM.  P.  SMITH, 
Preceptor  of  Mathematics  and  Natural  Philosophy^ 
No.  132,  South  Tenth  Street. 


Sir — I  have  carefully  examined  "  The  New  Federal  Calculator,  or  Scho- 
lar's Assistant,"  by  Thomas  T.  Smiley,  on  which  you  politely  requested  my 
opinion ;  and  freely  acknowledge  that  I  think  it  better  calculated  for  the  use 
of  the  United  States  schools  and  counting-houses  than  any  book  on  the  sub- 
ject that  I  have  seen.  The  author's  arrangement  of  the  four  primary  rules  is, 
in  my  opinion,  a  judicious  and  laudable  innovation,  claiming  the  merit  of  im- 
provement ;  as  it  brings  together  the  rules  nearest  related  in  their  nature  and 
uses.  His  queetions  upon  the  rules  throughout,  appear  to  me  to  be  admirably 
calculated  to  elicit  the  exertions  of  the  learner.  But  above  all,  the  preference 
he  has  given  to  the  currency  of  his  own  country,  in  its  numerous  examples, 
has  stamped  a  value  upon  this  little  work,  which  I  believe  Ijias  not  fallen  to 
the  lot  of  any  other  book  of  the  kind,  as  yet  offered  to  the  American  public 
I  am,  sir,  yours  respectfully, 

JOHN  MACKAY. 

Charleston,  [S.  C.)  March  29,  1825. 


From  the  United  States  Gazette. 

Among  the  numerous  publications  of  the  present  day,  devoted  to  the  im- 
provement of  youth,  ^^e  have  noticed  a  new  edition  of  Smiley's  Arithmetic, 
just  published  by  J.  Grigg. 

The  general  arrangement  of  this  book  is  an  improvement  upon  the  Arith- 
metics in  present  use,  being  more  systematic,  and  according  to  the  affinities  of 
different  rules.  The  chief  advantage  of  the  present  over  the  first  edition,  is  a 
correction  of  several  typographical  errors,  a  circumstance  which  will  render 
it  peculiarly  acceptable  to  teachers.  In  referring  to  the  merits  of  this  little 
work,  it  is  proper  to  mention  that  a  greater  portion  of  its  pages  are  devoted  to 


Federal  calculation,  than  is  generally  allows  in  primary  works  in  thjs  branch 
of  study.  The  heavy  tax  of  time  and  patience  which  our  youth  are  now 
compelled  to  pay  to  the  errors  of  their  ancestors,  by  performing  the  various 
operations  of  pounds,  shillings,  and  pence,  should  be  remitted,  and  we  are  glad 
to  notice  that  the  Federal  computation  is  becoming  the  prominent  practice  o^ 
school  arithmetic. 

In  recommending  Mr.  Smiley's  book  to  the  notice  of  parents  and  teachers, 
we  believe  that  we  invite  their  attention  to  a  work  that  will  really  prove  au 
"  assistant"  to  them,  and  a  "gwirfe"  to  their  interesting  charge. 


The  Editors  of  the  New  York  Telegraph,  speaking  of  Smiley's  Arithmetic, 
observe  that  they  have  within  a  few  days  attentively  examined  the  above 
Aritlimetic,  and  say,  "  We  do  not  hesitate  to  pronounce  it  an  improvement 
upon  every  work  of  the  kind  previously  before  the  public ; '  and  as  such,  re- 
commend its  adoption  in  all  our  Schools  and  Academies." 


A  KEY  to  the  above  Arithmetic,  in  which  all  the  Examples  ne- 
cessary for  a  Learner  are  wrought  at  large,  and  also  Solutions 
given  of  all  the  various  Rules.  Designed  principally  to  facili- 
tate the  labour  of  Teachers,  and  assist  such  as  have  not  the 
opportunity  of  a  tutor's  aid.  By  T.  T.  Smiley,  author  of  tlie 
New  Federal  Calculator,  Sac.  &c. 


TORREY'S  SPELLING  BOOK,  or  First  Book  for  Children. 

I  have  examined  Mr.  Jesse  Torrey's  "  Familiar  Spelling  Book."  I  think 
it  a  great  improvement  in  the  primitive,  and  not  least  important  branches  of 
education,  and  shall  introduce  it  into  the  seminaries  under  my  care,  as  one  su- 
perior to  any  which  has  yet  appeared. 

IRA  HtLL,  A.  M. 
Boonsboroughi  Feb.  2,  1825. 

The  increasing  demand  for  this  work  is  the  best  evidence  of  its  merits. 


A  PLEASING  COMPANION  FOR  LITTLE  GIRLS  AND 
BOYS,  blending  Instruction  with  Amusementj  being  a  Selec- 
tion of  Interesting  Stories,  Dialogues,  Fables,  and  Poetry. 
Designed  for  the  use  of  Primary  Schools  and  Domestic  Nur* 
series.     By  Jesse  Torrey,  jr. 

To  secure  the  perpetuation  of  our  republican  form  of  government  to  future 
generations,  let  Divines  and  Philosophers,  Statesmen  and  Patriots,  imite  their 
endeavours  to  renovate  the  age,  by  impressing  the  minds  of  the  people  with 
the  importance  of  educating  their  little  boys  and  girls.  S.  Adams. 


lY 


Report  of  the  Committee  of  the  Philadelphia  Academy  of  Teachers :  adopted 
J^ov.  6, 1824. 

The  Committee,  to  whom  was  referred  Mr.  Jesse  Torrey's  "  Pleasing, 
Companion  for  Little  Girls  and  Boys,"  beg;  leave  to  report, 

That  they  have  perused  the  "  Pleasing  Companion,"  and  have  much  plea- 
sure in  pronouncing  as  their  opinion,  that  it  is  a  compilation  much  better  cal- 
culated for  the  exercise  and  improvement  of  small  children  in  the  art  of  read- 
ing, and  especially  in  the  more  rare  art  of  understanding  what  they  read,  than 
the  books  in  general  use. 

All  which  is  respectfully  submitted. 

I.  IRVINE  HITCHCOCK, 
PARDOxN  DAVIS, 
CHARLES  MEAD, 

Committee. 
A  true  copy  from  the  miHutes  of  the  Academy. 

C.  B.  Trego,  Secretary. 
Nw.  22, 1824. 


THE  MORAL  INSTRUCTOR  AND  GUIDE  TO  VIRTUE, 

by  Jesse  Torrey,  Jr. 

Among  the  numerous  recommendations  to  this  valuable  School  Book,  are 
the  following : — 

Extract  of  a  note  from  the  Hon.   Thomas  Jefferson,  late  President  of  the 
United  States. 

"  I  thank  you,  sir,  for  the  copy  of  your  '  Moral  Instructor.^  I  have  read 
the  first  edition  with  great  satisfaction,  and  encouraged  its  reading  in  my 
family." 


Extracts  of  a  Letter  from  the  Hon.  James  Madison^  late  President  qf  the 
United  States. 

"  Sir — I  have  received  your  letter  of  the  15th,  with  a  copy  of  the  Moral 
Instructor. 

"  I  have  looked  enough  into  your  little  volume  to  be  satisfied,  that  both  the- 
original  and  selected  parts  contain  information  and  instruction  which  may  be 
useful,  not  only  to  juvenile  but  most  other  readers. 

"  With  friendly  respects, 

JAMES  MADISON. 
Dr.  Torrey. 


From  Roberts  Vaux^  President  of  the  Controllers  of  the  Public  Schools  in 
Philadelphia. 

"  The  Moral  Instructor"  is  a  valuable  compilation.     It  appears  to  be  well 


adapted  for  elementary  schools,  and  it  will  g^ve  me  pleasure  to  learn  that  the 
lessons  which  it  contains  are  furnished  for  the  improvement  of  our  youth  ge- 
nerally. Respectfully, 

,  ROBERTS  VAUX. 

Philadelphia,  5th  month,  8  1823. 


HISTORY  OF  ENGLAND,  from  the  First  Invasion  by  Julius 
Caesar,  to  the  Accession  of  George  the  Fourth,  in  eighteen 
hundred  and  twenty:  comprising  every  Political  Event  worthy 
of  remembrance;  a  Progressive  View  of  Religion,  Language, 
and  Manners;  of  Men  eminent  for  their  Virtue  or  their  Learn- 
ing; their  Patriotism,  Eloquence,  or  Philosophical  Research; 
of  the  Introduction  of  Manufactures,  and.  of  Colonial  Esta- 
blishments. With  an  interrogative  Index,  for  the  use  of 
Schools.  By  William  Grimshaw,  author  of  a  History  of  the 
United  States,  &c. 

HISTORY  OF  THE  UNITED  .STATES,  from  their  first 
settlement  as  Colonies,  to  the  cession  of  Florida,  in  1821: 
comprising  every  Important  Political  Event;  with  a  Progres- 
sive View  of  the  Aborigines;  Population,  Religion,  Agricul- 
ture, and  Commerce;  of  the  Arts,  Sciences,  and  Literature; 
occasional  Biographies  of  the  most  remarkable  Colonists, 
Writers,  and  Philosophers,  Warriors,  and  Statesmen;  and  a 
Copious  Alphabetical  Index.  By  William  Grimshaw,  author 
of  a  History  of  England,  &c. 

Also,  QUESTIONS  adapted  to  the  above  History,  and  a  KEY, 
adapted  to  the  Questions,  for  the  use  of  Teachers. 


"  University  of  Georgia,  Athens,  June  4, 1825. 
"Dear  Sir, 

"  With  grateful  pleasure,  I  have  read  the  two  small  volumes  of  Mr.  Grim- 
shaw, (a  History  of  England,  and  a  History  of  the  United  States)  which  you 
some  time  since  placed  in  my  hands.  On  a  careful  perusal  of  them,  I  feel  no 
difficulty  in  giving  m^  opinion,  that  they  are  both,  as  to  style  and  sentiment, 
works  of  uncommon  merit  in  their  kind ;  and  admirably  adapted  to  excite,  in 
youthful  minds,  the  love  of  historical  research. 

"  With  sincere  wishes  for  the  success  of  his  literary  labours, 

"  I  am  very  respectfully,  your  friend, 

"  M.  Waddel,  President. 
'^E.  Jacksobt,  Esa." 


D.  Javpoit  presents  his  respectful  compliments  to  Mr.  Grimshaw,  and  is 


VI 

much  obliged  by  his  polite  attention,  and  the  handsome  compliment  of  his 
History  of  the  United  States  with  the  Questions  and  Key. 

"  Mr.  J.  has  been  in  the  use  of  this  book  for  some  time ;  but  anticipates 
still  more  pleasure  to  himself,  and  profit  to  his  pupils,  in  future,  from  the  help 
and  facility  which  the  questions  and  key  will  afford  in  tne  study  of  these  in- 
teresting pages. 

"  October  10th,  1822." 


Golgotha,  P.  Edwd.  Fa.  Sep.  26,  1820. 
"  Dear  Sir, 

"  Mr.  Grimshaw's  *  History  of  the  United  States,'  &c.  was  some  time 

ago  put  into  my  hands  by  Mr.  B ,  who  requested  me  to  give  you  my 

opinion  as  to  the  merits  of  the  work.  The  history  of  the  late  war  is  well  manag- 
ed by  your  author:  it  has  more  of  detail  and  interest  than  the  former  part;  and 
I  consider  it  much  superior  to  any  of  the  many  compilations  on  that  subject, 
with  which  the  public  has  been  favoured.  It  may  be  said  of  the  entire  per- 
formance, that  it  is  decidedly  the  best  chronological  series,  and  the  chastest 
historical  narrative,  suited  to  the  capacity  of  the  juvenile  mind,  that  has  yet 
appeared.  Its  arrangement  is  judicious  ;  its  style  neat,  always  perspicuous, 
and  often  elegant ;  and  its  principles  sound. 

"  American  writings  on  men  and  things  connected  with  America,  have  been 
long  needed  for  the  young ;  and  I  am  happy  to  find,  that  Mr.  Grimshaw  has 
not  only  undertaken  to  supply  this  want,  but  also  to  Americanise  foreign  his- 
tory for  the  use  of  our  schools.  In  a  word,  sir,  I  am  so  fond  of  American  fa- 
brics, and  so  anxious  to  show  myself  humbly  instrumental  in  giving  our  youth 
American  feelings  and  character  whilst  at  school ;  that  1  shall  without  hesita- 
tion recommend  Mr.  Grimshaw's  Works  to  my  young  pupils,  as  introductory 
to  more  extensive  historical  reading.  In  fine,  the  work  is  so  unobjectionable, 
end  puts  so  great  a  mass  of  necessary  information  witliin  the  reach  of  school- 
boys, at  so  cheap  a  rate,  that  I  fee]  the  highest  pleasure  in  recommending  it  to 
the  public,  and  wish  you  extensive  sales. 

Yours  respectfully, 

"William  Branch,  Jr. 
"  Me.  Bxhtjamin  Warner, 

"  Philadelphia." 


*''  History  of  the  United  States,  from  their  first  settlement  as  Colonips^  to  the 
Peace  of  Ghent.,  Sec.     By  William  Grimshaw,  pp.  312,  12mo. 

"  This  is  the  third  time,  within  the  space  of  two  years,  that  we  have  had 
occasion  to  review  a  volume  from  the  hand  of  Mr.  Grimshaw.  He  writes 
with  gi-eat  rapidity ;  and  improves  as  he  advances.  This  is  the  most  cor- 
rectly written  of  all  his  productions.  We  could  wish  that  a  person  so  well 
formed  for  close,  and  persevering  study,  as  he  must  be,  might  find  encourage- 
ment to  davote  himself  to  the  interests  of  literature." 

"  Mr.  G.  has  our  thanks  for  the  best  concise  and  comprehensive  history  of 
the  United  States  which  we  have  seen." 

Theological  Review,  October^  1819. 


vu 

"  History  of  England,  from  the  first  Invasion  by  Julius  Cczsar^  to  the  Peace  qf 
Ghent,  Sec.  For  the  use  of  Schools.  By  William  Grimshaw.  Philadel- 
phia, 1819.     Benjamin  Warner.     12mo.  pp.  300. 

■  We  have  copied  so  much  of  the  title  of  this  work,  barely  to  express  our 
wcided  approbation  of  the  book,  and  to  recommend  its  general  introduction 
into  schools.  It  is  one  of  the  best  books  of  the  kind  to  be  found,  and  is  in- 
structive even  to  an  adult  reader.  We  should  be  pleased  that  teachers  would 
I  ;iak  it  among  tlieir  class-books ;  for  it  is  well  calculated  to  give  correct  im- 
j)ressi6ns,  to  its  readers,  of  the  gradual  progress  of  science,  religion,  govern- 
ment, and  many  other  institutions,  a  knowledge  of  which  is  beneficial  in  the 
present  age.  Among  the  many  striking  merits  of  this  book,  are,  the  perspi- 
cuity of  the  narrative,  and  chasteness  of  the  style.  It  is  with  no  little  pleasure 
we  have  learned,  that  the  author  has  prepared  a  similar  history  of  the  United 
States;  a  work  Icvng  wanted,  to  fill  up  a  deplorable  chasm  in  the  education  of 
American  youth." 

Analectic  Magazine^  October^  1819. 


•"  Philadelphia^  23  June,  1819. 
"  Sir — I  have  read  with  pleasure  and  profit  your  History  of  England.  I 
think  it  is  written  with  perspicuity,  chasteness,  and  impartiality.  Well  writ- 
ten history  is  the  best  political  instructor,  and  under  a  government  in  which  it 
is  the  blessing  of  the  country  that  the  people  govern,  its  pages  should  be  con- 
stantly in  the  hands  of  our  youtli,  and  lie  open  to  the  humblest  citizen  in  our 
wide-spread  territories.  Your  book  is  eminently  calculated  thus  to  diffuse 
this  important  knowledge,  and  therefore  entitled  to  extensive  circulation  ; 
which  I  most  cordially  wish.     With  much  respect, 

"  Your  obedient  servant, 

"IvANGDON   ChEVBS. 

"  William  Grimshaw,  Esa." 


A  NEW  METHOD  OF  TEACHING  THE  ART  OF  BOOK- 
KEEPING, by  the  use  of  1.  Necessary  Definitions  and  Uni- 
versal Rules ;  2.  Interrogatory  Exercises,  or  Oral  Journaliz- 
ing;  3.  Practical  Exercises,  accompanied  by  blank  books  and 
directions  for  using  them  ;  4.  Instructions  mr  the  adjustment 
and  closure  of  the  Leger,  the  re-opening  of  the  accounts  in 
the  old  books,  and  the  transfer  of  them  to  new  ones :  ac- 
companied by  a  Key,  by  the  assistance  of  which  Instructors 
are  enabled  to  teach  this  art  with  facility  and  success  to  youth 
of  proper  age  and  capacity,  and  adult  persons  to  acquire  a 
knowledge  of  it  without  the  help  of  a  teacher.  .  The  whole 
comprised  in  fifteen  Lessons,  and  the  Rules  and  Instructions 
exemplified  in  two  sets  of  books  kept  by  Double  Entry.  To 
wliich  are  added,  (in  the  Ke;y^,)  Specimens  showing  the  forma 
of  the  most  important  auxiliary  Books  connected,  as  such» 
witli  the  preceding  sets.  By  I.  Irvine  Hitchcock,  Accountant 
and  Teacher. 


via 

The  following  are  extracts  of  letters  concerning  the  above  entitled  work  by 
Accountants  and  Teachers  of  the  first  respectability  and  eminence  in  the  prin- 
cipal cities  of  the  United  States. 

"  I  do  not  hesitate  to  pronounce  Mr;  Hitchcock's  System  of  Book-keeping, 
in  my  opinion,  superior  to  any  other  treatise  I  have  yet  sden." 

"  We  are  convinced  that  in  point  of  utility  it  is  superior  to  any  other  that 
kas  hitherto  appeared." 

"  It  appears  to  me  to  excel  in  ease  and  perspicuity  every  other  system  with 
which  I  am  acquainted,  and  I  presume  it  needs  only  to  be  known  to  be  gene- 
rally adopted  in  our  schools." 

"  It  is  preferable  to  any  other  treatise  which  has  fallen  under  my  notice." 

"  We  are  decidedly  of  opinion,  that  it  is  better  calculated  to  give  correct  in- 
struction in  that  most  useful  art  than  any  other  work  which  we  have  seen." 

"  I  give  your  work  a  decided  preference  over  any  other  which  has  come  to 
my  knowledge." 

"  It  is  so  simple  that  most  persons,  even  without  an  instructor,  may,  by  di- 
ligence, acquire  a  competent  knowledge  of  the  art." 

"  I  deem  it  a  valuable  acquisition  for  the  use  of  schools." 

"I  freely  say,  that  both  the  plan  and  the  execution  of  the  work  (Hitch- 
cock's Book-keeping)  have  my  entire  approbation.  I  consider  it  a  useful  and 
valuable  acquisition  to  our  seminaries  of  learning,  and  highly  worthy  of  their 
patronage. 

J.  V.  N.  YATES, 
Acting  Swperintendant  of  Common  Schools^  for  fhe  State  of  New  York." 


"  This  system  has,  after  repeated  trials  and  comparisons,  been  pronounced 
superior  to  any  other  heretofore  published.  The  ease  with  which  the  learner 
finds  all  the  complex  parts  of  Book-keeping  explained,  has  astonished  the 
most  incredulous,  and  banished  the  idea  that  a  boy  must  attend  for  months  to 
learn  so  simple,  but  necessary  a  branch  of  education," 

Literary  Register, 


THE  UNITED  STATES  SPEAKER,  compiled  by  T.  T. 
Smiley — preferred  generally  to  the  Columbian  Orator  and 
Scott's  Lessons,  and  works  of  that  kind,  by  teachers  who 
have  examined  it. 

GOLDSMITH'S  HISTORY  OF  GREECE,  improved  by 
Grimshaw,  with  a  Vocabulary  of  the  Proper  Barnes  con- 
tained in  the  work,  and  the  Prosodial  Accents,  in  conformity 
with  the  Pronunciation  of  Lempriere — with  Questions  and  a 
Key,  as  above. 

GRIMSHAW'S  ETYMOLOGICAL  DICTIONARY  AND 
EXPOSITOR  OF  THE  ENGLISH  LANGUAGE. 


CONVERSATIONS 


ON 


NATURAL    PHILOSOPHY, 

IN    WHICH 

TBB  ZaZiSXHXENTS  OF  THAT  SCXENOS 

ARE    FAMILIARLY    EXPLAINED. 

BY  THE  AUTHOR  OF   CONVERSATIONS  ON  CHEMISTRY,  &C. 


WiTII  CORRECTIONS,  IMPROVEMENTS,  AND  CONSIDERABLE  ADDITIONS. 
IN  THE  BODY  OF  THE  VSTORK  ; 

Appropriate  (Questions,  ants  a  (HSilomavs: 

BY  DR.  THOMAS  P.  JONES, 

PROFESSOR     OF     MECHANICS,     IN    THE     FRANKLIN     INSTITUTf 
OF     THE     STATE     OF     PENNSYLVANIA. 


PHILADELPHIA  I 

PUBLISHED  AND  SOLD  BY  JOHN  GRIGG, 

NO.  9  NORTH  FOURTH  STREET. 
Stereotyped  by  L.  Johnson. 


1823. 


£astern  District  of  Pennsylvania^  to  toit : 

Be  it  remembered,  that,  on  the  twenty-fourth  day  of  April, 
in  the  Fiftieth  year  of  the  Independence  of  the  United  States  of  America, 
A.  D.  1826,  John  Grigg,  of  the  said  District,  hath  deposited  in  this  office  the 
title  of  a  book,  the  right  whereof  he  claims  as  proprietor,  ia  the  words  fol- 
lowing, to  wit : 

"'  Conversations  on  Natural  Philosophy,  in  which  the  Elements  of  that  Sci- 
ence are  familiarly  explained.  Illustrated  with  Plates.  By  the  Author  of 
Conversations  on  Chemistry,  &zc.  With  Corrections,  Improvements,  and 
considerable  Additions,  in  the  Body  of  the  Work;  appropriate  Questions, 
and  a  Glossary :  By  Dr.  Thomas  P.  Jones,  Professor  of  Mechanics,  in  the 
Franklin  Institute,  of  the  State  of  Pennsylvania." 

In  conformity  to  the  Act  of  the  Congress  of  the  United  States,  entitled, 
'*  An  Act  for  the  Encouragement  of  Learning,  by  securing  the  Copies  of 
Maps,  Charts,  and  Books,  to  the  Authors  and  Proprietors  of  such  Copies, 
during  the  times  therein  mentioned ;" — And  also  to  the  Act,  entitled, "  An  Act 
supplementary  to  an  Act,  entitled,  '  An  Act  for  the  Encouragement  of 
Learning,  by  securing  the  Copies  of  Maps,  Charts,  and  Books,  to  the  Au- 
thors and  Proprietors  of  such  Copies  during  the  times  tjjerein  mentioned,' 
and  extending  the  benefits  thereof  to  the  arts  of  designing,  engraving,  and 
etching,  historical  and  other  prints." 

D.  CALDWELL, 
Clerk  of  the  Eastern  District  of  Pennsylvania. 


PREFACE. 


Notwithstanding  the  great  number  of  books 
which  are  written,  expressly  for  the  use  of  schools, 
and  which  embrace  every  subject  on  which  in- 
struction is  given,  it  is  a  lamentable  fact,  that 
the  catalogue  of  those  which  are  well  adapted 
to  the  intended  purpose,  is  a  very  short  one.  Al- 
most all  of  them  have  been  written,  either  by  those 
who  are  without  experience  as  teachers,  or  by 
teachers,  deficient  in  a  competent  knowledge  of 
the  subjects,  on  which  they  treat.  Every  intelli- 
gent person,  who  has  devoted  himself  to  the  in- 
struction of  youth,  must  have  felt  and  deplored, 
the  truth  of  these  observations. 

In  most  instances,  the  improvement  of  a  work 
already  in  use,  will  be  more  acceptable,  than  one 
of  equal  merit  would  be,  which  is  entirely  new ; 
the  introduction  of  a  book  into  schools,  being  al- 
ways attended  with  some  difficulty. 

The  "Conversations  on  Chemistry,''  written 
by  Mrs.  Marcet,  had  obtained  a  well-merited 
celebrity,  and  was  very  extensively  adopted  as  a 
school-book,  before  the  publication  of  her  "  Con- 
versations on  Natural  JPhilosophy."  This,  also, 
has  been  much  used  for  the  same  purpose ;  but, 
the  observation  has  been  very  general,  among  intel- 
ligent teachers,  that,  in  its  execution,  it  is  very  in- 
ferior to  the  former  work. 

The  editor  of  the  edition  now  presented  to  the 
public,  had  undertaken  to  add  to  the  work,  ques- 
tions, for  the  examination  of  learners  ;  and  notes, 
where  he  deemed  them  necessary.  He  soon 
found,  however,  that  the  latter  undertaking  would 


IV  PREFACE. 


be  a  very  unpleasant  one,  as  he  must  have  pointed 
out  at  the  bottom  of  many  of  the  pages,  the  de- 
fects and  mistakes  in  the  text ;  whilst  numerous 
modes  of  illustration,  or  forms  of  expression, 
which  his  experience  as  a  teacher,  had  con- 
vinced him  would  not  be  clear  to  the  learner, 
must,  of  necessity,  have  remained  unaltered.  He 
therefore  determined  to  revise  the  whole  work,  and 
with  the  most  perfect  freedom,  to  make  such  al- 
terations in  the  body  of  it,  as  should,  in  his  opi- 
nion, best  adapt  it  to  the  purpose  for  which  it  was 
designed.  Were  the  book,  as  it  now  stands,  care- 
fully compared  with  the  original,  it  would  be 
found,  that,  in  conformity  with  this  determina- 
tion, scarcely  a  page  of  the  latter,  remains  un- 
changed. Verbal  alterations  have  been  made,  er- 
rors, m  points  of  fact,  have  been  corrected ;  and 
new  modes  of  illustration  have  been  introduced, 
whenever  it  was  thought  that  those  already  em- 
ployed, could  be  improved ;  or  when  it  was 
known,  that,  from  local  causes,  they  are  not  fami- 
liar, in  this  country. 

The  editor  feels  assured,  that,  in  performing 
this  task,  he  has  rendered  the  book  more  valuable 
to  the  teacher,  and  more  useful  to  the  pupil ;  and 
he  doubts  not  that  the  intelligent  author  of  it, 
would  prefer  the  mode  which  has  been  adopted, 
to  that  which  was  at  first  proposed. 

The  judicious  teacher  will,  of  course,  vary  the 
questions  according  to  circumstances;  and  those 
who  may  not  employ  them  at  all,  as  questions, 
will  still  find  them  useful,  in  directing  the  pupil 
to  the  most  important  points,  in  every  page. 

The  Glossary  has  been  confined  to  such  terms 
of  science  as  occur  in  the  work ;  and  is  believed  to 
include  all  those,  of  which  a  clear  definition  can- 
not be  found  in  our  common  dictionaries. 


CONTENTS. 


CONVERSATION  I. 

Page 

ON   GENERAL   PROPERTIES   OP   BODIES.  9 

Introduction. — General  Properties  of  Bodies. — Impenetrability. — Exten- 
sion.— Figure. — Divisibility. — Inertia. — Attraction.— Attraction  of  Cohe- 
sion.— Density. — Rarity. — Heat. — Attraction  of  Gravitation. 


CONVERSATION  II. 

ON   THE   ATTRACTION   OF    GRAVITY.  S 

Attraction  of  Gravitation,  continued. — Of  Weight. — Of  the  Fall  of  Bodies.- 
Of  the  Resistance  of  the  Air. — Of  the  Ascent  of  Light  BoJies. 


CONVERSATION  III. 

ON   THE   LAWS   OF   MOTION.  32 

Of  Motion. — Of  the  Inertia  of  Bodies. — Of  Force  to  produce  Motion. — Di- 
rection of  Motion. — Velocity,  absolute  and  relative. — Uniform  Motion. — 
Retarded  Motion.— Accelerated  Motion, — Velocity  of  Falling  Bodies. — 
Momentum. — Action  and  Reaction  equal. — Elasticity  of  Bodies. — Porosity 
of  Bodies. — Reflected  Motion^ — Angles  of  Incidence  and  Reflection. 

CONVERSATION  IV. 

ON  COMPOUND  MOTION.  46 

Compound  Motion,  the  result  of  two  opposite  forces. — Of  Curvilinear  Motion, 
the  result  of  two  forces. — Centre  of  Motion,  the  point  at  rest,  while  the 
other  parts  of  the  body  move  round  it. — Centre  of  Magnitude,  the  middle 
of  a  body. — Centripetal  Force,  that  which  impels  a  body  towards  a  fixed 
central  point. — Centrifugal  Force,  that  which  impels  a  body  to  fly  from 
the  centre. — Fall  of  Bolies  in  a  Parabola. — Centre  of  Gravity,  the  point 
about  which,  the  parts  balance  each  other, 
A2 


vi                                                          CONTENTSi  \ 

Page  • 

CONVERSATION  V.  ^ 

ON    THE   MECHANICAL   POWERS.                                        54  .] 

Of  the  Power  of  Machines. — Of  the  Lever  in  general. — Of  the  Lever  of  the  | 
first  kind,  having  tlie  Fulcrum  between  tlie  power  and  the  weight. — Of  the 

Lever  of  the  second  kind,  having  the  Weight  between  the  power  and  the  \ 

fulcrum. — Of  the  Lever  of  the  third  kind,  having  the  Power  between  the  : 

fulcrum  and  the  weight.— Of  the  Pulley.— Of  the  Wheel  and  Axle.— Of  J 

the  Inclined  Plane. Of  the  Wedge. — Of  the  Screw.  j 

CONVERSATION  VI.  i 

ASTRONOMY.  1 

CAUSES  OF*  THE  MOTION  OF  THE   HEAVENLY    BODIES.               70  \ 

\ 

Of  the  Earth's  annual  motion. — Of  the  Planets,  and  their  motion. — Of  the 

Diurnal  motion  of  the  Earth  and  Planets.  ; 

1 

CONVERSATION  VII.  \ 

ON   THE   PLANETS.                                                           80  j 

Of  the  Satellites  and  Moons. — Gravity  diminishes  as  the  Square  of  the  Dis-  \ 

tance. — Of  the  Solar  System. — Of  Comets. — Constellatioos,  signs  of  the  ^ 

Zodiac. — Of  Copernicus,  Newton,  &c.  1 

CONVERSATION  VIII.  ^ 

ON   THE   EARTH.  91 

Of  the  Terrestrial  Globe.— Of  the  Figure  of  the  Earth.— Of  the  Pendulum. 
— Of  the  Variation  of  the  Seasons,  and  of  the  Length  of  Days  and  Nights. — 
Of  the  Causes  of  tlie  Heat  of  Summer. — Of  Solar,  Sideral,  and  Equal  or 
Mean  Time. 


CONVERSATION  IX.  j 

ON   THE    MOON.                                                            108  \ 

Of  the  Moob's  Motion.— Phases   of  the  Moon.— Eclipses  of  the  Moon.—  ] 

Eclipses  of  Jupiter's  Moons. — Of  Latitude  and  Longitude. — Of  the  Tran&its  J 

of  the  inferior  Planets. — Of  the  Tides.  | 


CONTENTS.  VH 

Pagi 
CONVERSATION  X. 

HTDROSTATICS. 
ON  THE  MECHANICAL  PROPERTIES  OF    FLUIDS.  118 

Definition  of  a  Fluid. — Distinction  between  Fluids  and  Liquids. — Of  Non- 
Elastic  Fluids,  scarcely  susceptiblq  of  Compression. — Of  the  Cohesion  of 
Fluids. — Of  their  Gravitation. — Of  tlieir  Equilibrium. — Of  their  Pressure. 
— Of  Specific  Gravity. — Of  the  Specific  Gravity  of  Bodies  heavier  than 
"Water. — Of  those  of  the  same  weight  as  Water. — Of  those  lighter  than 
Water. — Of  the  Specific  Gravity  of  Fluids. 

CONVERSATION  XI. 

OF  SPRINGS,   FOUNTAINS,  &C.  128 

Of  the  Ascent  of  Vapour  and  the  Formation  of  Clouds. — Of  the  Formation 
and  Fall  of  Rain,  &c — Of  the  Formation  of  Springs. — Of  Rivers  and  Lakes. 
— Of  Fountains. 

CONVERSATION  XII. 

PNEUMATICS. 

ON  THE   MECHANICAL  PROPERTIES  OF  AIR.  136 

Of  the  Spring  or  Elasticity  of  the  Air. — Of  the  Weight  of  the  Air. — Experi- 
ments with  the  Air  Pump. — Of  the  Barometer. — Mode  of  Weighing  Air. 
—Specific  Gravity  of  Air. — Of  Pumps. — Description  of  the  Sucking  J'ump. 
—Description  of  the  Forcing  Pump. 

CONVERSATION  XIII. 

ON   WIND  AND   SOUND.  146 

Of  Wind  in  General. — Of  tlie  Trade  Wind. — Of  the  Periodical  Trade 
Winds. — Of  the  Aerial  Tides. — Of  Sound  in  General.— Of  Sonorous  Bo- 
dies.— Of  Musical  Sounds.— Of  Concord  or  Harmony,  and  Melody. 

CONVERSATION  XIV. 

ON  OPTICS.'  157 

Of  Luminous,  Transparent,  and  Opaque  Bodies. — Of  the  Radiation  of  Light. 
— Of  Shadows. — Of  the  Reflection  of  Light.— -Opaque  Bodies  seen  only 
by  Reflected  Light. — Vision  Explained. — Camera  Obscura. — Image  of 
Objects  on  the  Retina. 


i 


Ylll  CONTENTS. 

CONVERSATION  XV. 
OPTICS — continued. 

OF   THE  ANGLE   OF   VISION,  AND    REFLECTION  OF   MIRRORS.        168 

Angle  of  Vision. — Reflection  of  Plain  Mirrors. — Reflection  of  Convex  Mir- 
rors.—Reflection  of  Concave  Mirrors. 

CONVERSATION  XVI. 

ON  REFRACTION  AND   COLOURS.  179 

Transmission  of  Light  by  Transparent  Bodies. — Refraction. — Refraction  by 
the  Atmosphere. — Refraction  by  a  Lens. — Refraction  by  the  Prism. — Of 
Colour  from  the  Rays  of  Light. — Of  the  Colours  of  Bodies. 

CONVERSATION  XVII. 

ON  THE   STRUCTITRE   OF  THE  EYE,  AND   OPTICAL  INSTRUMENTS.     195 

Description  of  the  Eye. — Of  the  Image  on  the  Retina.— Refraction  by  the 
Humours  of  the  Eye.— Of  the  use  of  Spectacles. — Of  the  Single  Micro- 
scope.— Of  the  Double  Microscope.— Of  the  Solar  Microscope.— -Magic 
Lanthorn.— Refracting  Telescope. — Reflecting  Telescope. 

Glossary, 205 


CONVERSATION  I. 


ON  GENERAL  PROPERTIES  OF  BODIES. 

IJNTRODUCTIOW. — GENERAL  PROPERTIES  OF  BODIES. — IMPENETRABILITY. 
— EXTENSION. — FIGURE. — DIVISIBILITY. — INERTIA. — ATTRACTION. — 
ATTRACTION  OF  COHESION. — DENSITY. — RARUTY. — HEAT. — ATTRAC- 
TION OF  GRAVITATION. 

EMILY. 

I  MUST  request  your  assistance,  my  Dear  Mrs.  B.  in  a  charge 
which  I  have  lately  undertaken:  it  is  that  of  > instructing  my 
youngest  sister,  a  task,  which  I  find  proves  more  difficult  than  I 
had  at  first  imagined.  I  can  teach  her  the  common  routine  of 
children's  lessons  tolerably  well ;  but  she  is  such  an  inquisitive 
little  creature,  that  she  is  not  satisfied  without  an  explanation  of 
every  difficulty  that  occurs  to  her,  and  frequently  asks  me 
questions  which  I  am  at  a  loss  to  answer.  This  morning,  for 
instance,  when  I  had  explained  to  her  that  the  world  was  round 
like  a  ball,  instead  of  being  flat  as  she  had  supposed,  and  that  it 
was  surrounded  by  the  air,  she  asked  me  what  supported  it.  I 
told  her  that  it  required  no  support ;  she  then  inquired  why  it 
did  not  fall  as  every  thing  else  did  ?  This  I  confess  perplexed 
me ;  for  I  had  myself  been  satisfied  with  learning  that  the  world 
floated  in  the  air,  without  considering  how  unnatural  it  was  that 
so  heavy  a  body,  bearing  the  weight  of  all  other  things,  should  be 
able  to  support  itself. 

Mrs.  B.  I  make  no  doubt,  my  dear,  but  that  I  shall  be  able 
to  explain  this  difficulty  to  you ;  but  I  believe  that  it  would  be 
almost  impossible  to  render  it  intelligible  to  the  -comprehension 
of  so  young  a  child  as  your  sister  Sophia.  You,  who  are  now 
in  your  thirteenth  year,  may,  I  think,  with  great  propriety,  learn 
not  only  the  cause  of  this  particular  fact,  but  acquire  a  general 
knowledge  of  the  laws  by  which  the  natural  world  is  governed. 

Emily.  Of  all  things,  it  is  what  I  should  most  like  to  learn; 
but  I  was  afraid  it  was  too  difficult  a  study  even  at  my  age. 

Mrs.  B.  Not  when  familiarly  explained  :  if  you  have  patience 
to  attend,  I  will  most  willingly  give  you  all  the  information  in 
my  power.  You  may  perhaps  find  the  subject  rather  dry  at  first ; 


10  GENERAL    PROPERTIES    OF    BODIES. 

but  if  I  succeed  in  explaining  the  laws  of  nature,  so  as  to  make 
you  understand  them,  I  am  sure  that  you  will  derive  not  only 
instruction,  but  ffreat  amusement  from  that  study. 

Emily.  I  make  no  doubt  of  it,  Mrs.  B.;  and  pray  begin  by 
explaining  why  the  earth  requires  no  support;  for  that  is  the 
point  which  just  now  most  strongly  excites  my  curiosity. 

Mrs.  B.  My  dear  Emily,  if  I  am  to  attempt  to  give  you  a 
general  idea  of  the  laws  of  nature,  which  is  no  less  than  to  intro- 
duce you  to  a  knowledge  of  the  science  of  natural  philosophy,  it 
will  be  necessary  for  us  to  proceed  with  some  degree  of  regu- 
larity. I  do  not  wish  to  confine  you  to  the  systematic  order  of  a 
scientific  treatise,  but  if  we  were  merely  to  examine  every  vague 
question  that  may  chance  to  occur,  our  progress  would  be  but 
very  slow.  Let  us,  therefore,  begin  by  taking  a  short  survey  of 
the  general  properties  of  bodies,  some  of  which  must  necessarily 
be  explained  before  I  can  attempt  to  make  you  understand  why 
the  earth  requires  no  support. 

When  I  speak  of  bodies,  I  mean  substances,  of  whatever  na- 
ture, whether  solid  or  fluid  ;  and  matter  is  the  general  term  used 
to  denote  the  substance,  whatever  its  nature  be,  of  which  the 
different  bodies  are  composed.  Thus,  tlie  wood  of  which  this 
table  is  made,  the  water  with  which  this  glass  is  filled,  and  the 
air  which  we  continually  breathe,  are  each  of  them  matter. 

Emily.  I  am  very  glad  you  have  explained  the  meaning  of 
the  word  matter,  as  it  nas  corrected  an  errroneous  conception  I 
had  formed  of  it :  I  thought  that  it  was  applicable  to  solid  bodies 
only. 

Mrs.  B.  There  are  certain  properties  which  appear  to  be 
common  to  all  bodies,  and  are  hence  called  the  essential  or  inh^ 
rent  properties  of  bodies ;  these  are  Impenetrability,  Extensioii, 
Figure,  Divisibility,  Inertia  and  .Attraction.  These  are  also  called 
the  general  properties  of  bodies,  as  we  do  not  suppose  any  body 
to  exist  without  them. 

By  impenetrability  is  meant  the  property  which  bodies  have 
of  occupying  a  certain  space,  so  that  where  one  body  is,  another 
can  not  be,  without  displacing  the  former ;  for  two  bodies  can  not 
exist  in  the  same  place  at  the  same  time.  A  liquid  may  be  more 
easily  removed  than  a  solid  body;  yet  it  is  not  the  less  substan- 
tial, since  it  is  as  impossible  for  a  liquid  and  a  solid  to  occupy 
the  same  space  at  the  same  time,  as  for  two  solid  bodies  to  do 
so.  For  instance,  if  you  put  a  spoon  into  a  glass  full  of  water, 
the  water  will  flow  over  to  make  room  for  the  spoon. 

Emily.    I  understand  this  perfectly.     Liquids  are  in  reality 

1.  What  is  intended  by  the  term  bodies?  2.  Is  the  term  matter^  restrict- 
ed to  substances  of  a  particular  kind  ?  3.  Name  those  properties  of  bodies, 
which  are  called  inherent.  4.  What  is  meant  by  impenetrability  ?  5.  Can 
a  liquid  be  said  to  be  impenetrable? 


GENERAL    PROPERTIES    OF    BODIES.  11 

as  substantial  or  as  impenetrable  as  solid  bodies,  and  they  ap- 
pear less  so,  only  because  they  are  more  easily  displaced. 

Mrs.  B,  The  air  is  a  fluid  differing  in  its  nature  from  liquids, 
but  no  less  impenetrable.  If  I  endeavour  to  fill  this  phial  by 
plunging  it  into  this  bason  of  water,  the  air,  you  see,  rushes  out 
of  the  phial  in  bubbles,  in  order  to  make  way  for  the  water,  for 
the  air  and  the  water  can  not  exist  together  in  the  same  space, 
any  more  than  two  hard  bodies ;  and  if  I  reverse  this  goblet,  and 
plunge  it  perpendicularly  into  the  water,  so  that  the  air  will  not 
be  able  to  escape,  the  water  will  no  longer  be  able  to  fill  the 
goblet. 

Emily.    But  it  rises  some  way  into  the  glass. 

Mrs.  B.  Because  the  water  compresses  or  squeezes  the  air 
into  a  smaller  space  in  the  upper  part  of  the  glass ;  but,  as  long 
as  it  remains  there,  no  other  body  can  occupy  the  same  place. 

Emily.  A  difficulty  has  just  occurred  to  me,  with  regard  to  the 
impenetrability  of  solid  bodies  ;  if  a  nail  is  driven  into  a  piece  of 
wood,  it  penetrates  it,  and  both  the  wood  and  the  nail  occupy 
the  same  space  that  the  wood  alone  did  before  ? 

Mrs.  B.  The  nail  penetrates  between  the  particles  of  the 
wood,  by  forcing  them  to  make  way  for  it ;  for  you  know  that 
not  a  single  atom  of  wood  can  remain  in  the  space  which 
the  nail  occupies ;  and  if  the  wood  is  not  increased  in  size  by  the 
addition  of  the  nail,  it  is  because  wood  is  a  porous  substance, 
like  sponge,  the  particles  of  which  may  be  compressed  or  squeez- 
ed closer  together ;  and  it  is  'thus  that  they  make  way  for  the 
nail. 

We  may  now  proceed  to  the  next  general  property  of  bodies, 
extension.  A  body  which  occupies  a  certain  space  must  neces- 
sarily have  extension  ;  that  is  to  say,  length,  breadth  and  depth  or 
thickness ;  these  are  called  the  dimensions  of  extension :  can  you 
form  an  idea  of  any  body  without  them  ? 

Emily.  No;  certainly  I  can  not;  though  these  dimensions 
must,  of  course  vary  extremely  in  diflferent  bodies.  The  length, 
breadth  and  depth  of  a  box,  or  of  a  thimble,  are  very  different 
from  those  of  a  walking  stick,  or  of  a  hair. 

But  is  not  height  also  a  dimension  of  extension  ? 

Mrs  B.  Height  and  depth  are  the  same  dimension,  consider- 
ed in  different  points  of  view ;  if  you  measure  a  body,  or  a  space^ 
from  the  top  to  the  bottom,  you  call  it  depth ;  if  from  the  bottom 

6.  How  can  you  prove  that  air  is  impenetrable  ?  7.  If  air  is  impenetra- 
ble, what  causes  the  Water  to  rise  some  way  into  a  goblet,  if  I  plunge  it  iiito 
water  with  its  mouth  downward?  8.  When  I  drive  a  nail  into  wood,  do  not 
both  the  iron  and  the  wood  occupy  the  same  space?  9.  In  how  many  direc- 
tions is  a  body  said  to  bave  extension  ?  ]  0.  How  do  we  distinguish  the  terms 
height  and  depth? 


12  GENERAL    PROPERTIES    OF    BODIES. 

upwards,  you  call  it  height ;  thus  the  depth  and  height  of  a  box 
are,  in  fact,  the  same  thing. 

Emily.  Very  true;  a  moment's  consideration  would  have 
enabled  me  to  discover  that ;  and  breadth  and  width  are  also  the 
same  dimension. 

Mrs.  B.  Yes;  the  limits  of  extension  constitute  figure  or 
shape.  You  conceive  that  a  body  having  length,  breadth  and 
depth,  can  not  be  without  form,  either  symmetrical  or  irregular  ? 

Emily.  Undoubtedly ;  and  this  property  admits  of  almost  an 
infinite  variety. 

Mrs.  B.  Nature  has  assigned  regular  forms  to  many  of  her 
productions.  The  natural  form  of  various  mineral  substances 
IS  that  of  crystals,  of  which  there  is  a  great  variety.  Many  of 
them  are  very  beautiful,  and  no  less  remarkable  by  their  trans- 
parency or  colour,  than  by  the  perfect  regularity  of  their  forms, 
as  may  be  seen  in  the  various  museums  and  collections  of  natu- 
ral history.  The  vegetable  and  animal  creation  appears  less 
symmetrical,  but  is  still  more  diversified  in  figure  than  the  mine- 
ral kingdom.  Manufactured  substances  assume  the  various 
arbitrary  forms  which  the  art  of  man  designs  for  them ;  and  an 
infinite  number  of  irregular  forms  are  produced  by  fractures  and 
by  the  dismemberment  of  the  parts  of  bodies. 

Emily.     Such  as  a  piece  of  broken  china,  or  glass  ? 

Mrs.  B.  Or  the  masses  and  fragments  of  stone,  and  other  mine- 
ral substances,  which  are  dug  out  of  the  earth,  or  found  upon  its 
surface ;  many  of  which,  although  composed  of  minute  crystals., 
are  in  the  lump  of  an  irregular  form. 

We  may  now  proceed  to  divisibility;  that  is  to  say,  a  suscep- 
tibility of  being  divided  into  an  indefinite  number  of  parts.  Take 
any  small  quantity  of  matter,  a  grain  of  sand  for  instance,  and 
cut  it  into  two  parts ;  these  two  parts  miglit  be  again  divided, 
had  we  instruments  sufiiciently  fine  for  the  purpose ;  and  if  by 
means  of  pounding,  ginnding,  and  other  similar  methods,  we  car- 
ry this  division  to  the  greatest  possible  extent,  and  reduce  the 
body  to  its  finest  imaginable  particles,  yet  not  one  of  the  parti- 
cles will  be  destroyed,  but  will  each  contain  as  many  halves  and 
quarters,  as  did  the  whole  ^rain. 

The  dissolving  of  a  solid  body  in  a  liquid,  affords  a  very 
striking  example  of  the  extreme  divisibility  of  matter ;  when  you 
sweeten  a  cup  of  tea,  for  instance,  with  what  minuteness  the 
sugar  must  be  divided  to  be  diffused  throughout  the  whole  of  the 
liquid. 

11.  What  constitutes  \\\e  Jigure^  or  form  of  a  body?  12.  What  is  said 
respecting  the  form  of  minerals?  13.  What  of  the  vegetable  and  animal 
creation?  14.  What  of  artificial,  and  accidental  forms?  15.  What  is  meant 
by  divisibility?  16.  What  examples  can  you  give,  to  prove  that  the  parti- 
cles of  a  body  are  minute  in  the  extreme? 


GENERAL    PROPERTIES    OF    BODlSS»  IS 

Emily,  And  if  you  pour  a  few  drops  of  red  wine  into  a  glass 
of  water,  they  immediately  tinge  tlie  whole  of  the  water,  and 
must  therefore  be  diffused  throughout  it. 

Mrs.  B,  Exactly  so ;  and  tlie  perfume  of  this  lavender  water 
will  be  almost  as  instantaneously  diffused  throughout  the  room, 
if  I  take  out  the  stopper. 

Emily.  But  in  this  case  it  is  only  the  perfume  of  the  laven- 
der, and  not  the  water  itself  that  is  diffused  in  the  room. 

Mrs.  B.  The  odour  or  smell  of  a  body  is  part  of  the  body 
itself,  and  is  produced  by  very  minute  particles  or  exhalations 
which  escape  from  the  odoriferous  bodies.  It  would  be  impossi- 
ble that  you  should  smell  the  lavender  water,  if  particles  of  it 
did  not  come  in  actual  contact  with  your  nose. 

Emily.  But  when  I  smell  a  flower,  I  see  no  vapour  rise  from 
it;  and  yet  I  perceive  the  smell  at  a  considerable  distance. 

Mrs.  B.  You  could,  I  assure  you,  no  more  smell  a  flower,  the 
odoriferous  particles  of  which  did.  not  touch  your  nose,  than  you 
could  taste  a  fruit,  the  flavoured  particles  of  which  did  not  come 
in  c(mtact  with  your  tongue. 

Emily.  That  is  wonderful  indeed ;  the  particles  then,  which 
exhale  from  the  flower  and  from  the  lavender  water,  are,  I  sup- 
pose, too  small  to  be  visible  ? 

Mrs.  B.  Certainly:  you  may  form  some  idea  of  their  extreme 
minuteness,  from  the  immense  number  which  must  have  escaped 
in  order  to  perfume  the  whole  room ;  and  yet  there  is  no  sensible 
diminution  of  the  liquid  in  the  phial. 

Emily.     But  the  quantity  must  really  be  diminished  ? 

Mrs.  B.  Undoubtedly ;  and  were  you  to  leave  the  bottle  open 
a  sufficient  length  of  time,  the  whole  of  the  water  would  evapo- 
rate and  disappear.  But  though  so  minutely  subdivided  as  to  be 
imperceptible  to  any  of  our  senses,  each  particle  would  continue 
to  exist ;  for  it  is  not  within  the  power  of  man  to  destroy  a  single 
particle  of  matter :  not-  is  there  any  reason  to  suppose  that  in 
nature  an  atom  is  ever  annihilated. 

Emily.  Yet,  when  a  body  is  burnt  to  ashes,  part  of  it,  at  least, 
appears  to  be  effectually  destroyed  :  look  how  small  is  the  resi- 
due of  ashes  in  the  fire  place,  from  all  the  fuel  which  has  been 
consumed  within  it. 

Mrs.  B.  That  part  of  the  fuel,  which  you  suppose  to  be  de- 
stroyed, evaporates  in  the  form  of  smoke,  and  vapour,  and  air, 
whilst  the  remainder  is  reduced  to  ashes.  A  body,  in  burning, 
undergoes  no  doubt  very  remarkable  changes;  it  is  generally 
subdivided;  its  form  and  colour  altered;  its  extension  increased: 
but  the  various  parts,  into  which  it  has  been  separated  by  com- 

17.  What  produces  the  odour  of  bodies?  18.  How  do  odours  exemplify  the 
minuteness  of  the  particles  of  matter?  19.  Can  matter  be  in  any  way  anni- 
hilated ?     20.  What  becomes  of  the  fuel,  which  disappears  in  our  iii-es  ? 

B 


14  GENERAL    PROPERTIES    OF    BODIES. 

bustion,  continue  in  existence,  and  retain  all  the  essential  pro- 
perties of  bodies. 

Emily.  But  that  part  of  a  burnt  body  which  evaporates  in 
smoke  has  no  figure ;  smoke,  it  is  true,  ascends  in  columns  into 
the  air,  but  it  is  soon  so  much  diffused  as  to  lose  all  form ;  it 
becomes  indeed  invisible. 

Mrs.  B.  Invisible,  I  allow ;  but  we  must  not  imagine  that 
what  we  no  longer  see  no  longer  exists.  Were  every  parti- 
cle of  matter  that  becomes  invisible  annihilated,  the  world  itself 
would  in  the  course  of  time  be  destroyed.  The  particles  of 
smoke,  when  diffused  in  the  air,  continue  still  to  be  particles  of 
matter  as  well  as  when  more  closely  united  in  the  form  of  coals: 
they  are  really  as  substantial  in  the  one  state  as  in  the  other, 
and  equally  so  when  by  their  extreme  subdivision  they  become 
invisible.  No  particle  of  matter  is  ever  destroyed :  this  is  a  prin- 
ciple you  must  constantly  remember.  Every  thing  in  nature 
decays  and  corrupts  in  the  lapse  of  time.  We  me,  and  our 
bodies  moulder  to  dust;  but  not  a  single  atom  of  them  is  lost; 
they  serve  to  nourish  the  earth,  whence,  while  living,  they  drew 
their  support. 

The  next  essential  property  of  matter  is  called  inertia  or  in- 
activity ;  this  \n)rd  expresses  the  resistance  which  matter  makes 
to  a  change  from  a  state  of  rest,  to  that  of  motion,  or  from  a  state 
of  motion  to  that  of  rest.  Bodies  are  equally  incapable  of  chang- 
ing their  actual  state,  whether  it  be  of  motion  or  of  rest.  You 
know  that  it  requires  force  to  put  a  body  which  is  at  rest  in  mo- 
tion; an  exertion  of  strength  is  also  requisite  to  stop  a  body 
which  is  already  in  motion.  The  resistance  of  the  body  to  a 
change  of  state,  in  either  case,  arises  from  its  inertia. 

Emily.  In  playing  at  base-ball  I  am  obliged  to  use  all  tny 
strength  to  give  a  rapid  motion  to  the  ball;  and  when  I  have  to 
catch  it,  1  am  sure  I  feel  the  resistance  it  makes  to  being  stopped. 
But  if  I  did  not  catch  it,  it  would  soon  fall  to  the  ground  and  stop 
of  itself. 

Mrs.  B.  Matter  being  inert  it  is  as  incapable  of  stopping  of 
itself  as  it  is  of  putting  itself  into  motion :  when  the  ball  ceases 
to  move,  therefore,  it  must  be  stopped  by  some  other  cause  or 
power ;  but  as  it  is  one  with  which  you  are  yet  unacquainted,  we 
can  not  at  present  investigate  its  effects. 

The  last  property  which  appears  to  be  common  to  all  bodies 
is  attraction.  All  bodies  consist  of  infinitely  small  particles  of 
matter,  each  of  which  possesses  the  power  oi  attracting  or  draw- 
ing towards  it,  and  uniting  with  any  other  particle  sufficiently 

21.  How  can  that  part  which  evaporates,  be  still  said  to  possess  a  substan- 
tial form?  22.  What  do  we  mean  hy  inertia?  23.  Give  an  example  to 
prove  that  force  is  necessary,  either  to  give  or  to  stop  motion.  24.  What  ge- 
neral power  do  the  particles  of  matter  exert  upon  other  particles .'' 


GENERAL   PROPERTIES    OF    BODIES.  15 

near  to  be  within  the  influence  of  its  attraction ;  but  in  minute 
particles  this  power  extends  to  so  very  small  a  distance  around 
them,  that  its  effect  is  not  sensible,  unless  they  are  (or  at  least 
appear  to  be)  in  contact ;  it  then  makes  them  stick  or  adhere 
together,  and  is  hence  called  the  attraction  of  cohesion.  With- 
out this  power,  solid  bodies  would  fall  in  pieces,  or  rather  crum 
ble  to  atoms. 

Emily.  I  am  so  much  accustomed  to  see  bodies  firm  and  so- 
lid, that  it  never  occurred  to  me  that  any  power  was  requisite  to 
unite  the  particles  of  which  they  are  composed.  But  the  attrac- 
tion of  cohesion  dpes  not,  I  suppose,  exist  in  liquids;  for  the 
particles  of  liquids  do  not  remain  together  so  as  to  form  a  body, 
unless  confined  in  a  vessel  ? 

Mrs.  B.  Fbeg  your  pardon;  it  is  the  attraction  of  cohesion 
which  holds  this  drop  of  water  suspended  at  the  end  of  my  fin- 
ger, and  keeps  the  minute  watery  particles  of  which  it  is  com- 
posed unitedr  But  as  this  power  is  stronger  in  proportion  as  the 
particles  of  bodies  are  more  closely  united,  the  cohesive  attrac- 
tion of  solid  bodies  is  much  greater  than  that  of  fluids. 

The  thinner  and  lighter  a  fluid  is,  the  less  is  the  cohesive  at- 
traction of  its  particles,  because  they  are  further  apart ;  and  in 
elastic  fluids,  such  as  air,  there  is  no  cohesive  attraction  among 
the  particles. 

Emily.  That  is  very  fortunate ;  for  it  would  be  impossible  to 
breathe  the  air  in  a  solid  mass ;  or  even  in  a  liquid  state. 

But  is  the  air  a  body  of  the  same  nature  as  other  bodies  ? 

Mrs.  B.    Undoubtedly,  in  all  essen<tial  properties. 

Em,ily.  Yet  you  say  that  it  does  not  possess  one  of  the  gene- 
ral properties  of  bodies — attraction. 

Mrs.  B.  The  particles  of  air  are  not  destitute  of  the  power 
of  attraction,  but  they  are  too  far  distant  from  each  other  to  be 
influenced  by  it  so  as  to  produce  cohesion :  and  the  utmost  efforts 
of  human  art  have  proved  ineffectual  in  the  attempt  to  compress 
them,  so  as  to  bring  them  within  the  sphere  of  each  other's  at- 
traction, and  make  them  cohere. 

Emily.  .  If  so,  how  is  it  possible  to  prove  that  they  are  endow- 
ed with  this  power 't 

Mrs.  B.  The  air  is  formed  of  particles  precisely  of  the  same 
nature  as  those  which  enter  into  the  composition  of  liquid  and  solid 
bodies,  in  each  of  which  we  have  a  proof  of  their  attraction. 

Emily.  It  is  then,  I  suppose,  owing  to  the  different  decrees 
of  cohesive  attraction  in  different  substances,  that  they  areTiard 
or  soft,  and  that  liquids  are  thick  or  thin. 

25.  What  13  that  species  of  attraction  called,  which  keeps  bodies  in  a  solid 
state  ?  26.  Does  the  attraction  of  cohesion  exist  in  liquids,  and  how  is  its  ex- 
istence proved  ?  27.  If  the  particles  of  air  attract  each  other,  why  do  they 
not  cohere.'  28.  From  what  then  do  you  infer  that  they  possess  attraction.' 
29.  How  do  you  account  for  some  bodies  bein^  hard  and  others  soft  ? 


16  GENERAL   PROPERTIES    OF   BODIES. 

Mrs.  B.  Yes ;  but  }rou  would  express  your  meaning  better 
by  the  term  density,  wmch  denotes  trie  degree  of  closeness  and 
compactness  of  the  particles  of  a  body.  In  philosophical  lan- 
guage, density  is  said  to  be  that  property  of  bodies  hy  which 
they  contain  a  certain  quantity  of  matter,  under  a  certain  bulk 
or  magnitude.  Rarity  is  the  contrary  of  density;  it  denotes  the 
thinness  and  subtilty  of  bodies :  thus  jom  would  say  that  mercu- 
ry or  quicksilver  was  a  very  dense  fluid ;  ether,  a  very  rare  one. 
Those  bodies  which  are  the  most  dense,  do  not  always  cohere 
the  most  strongly ;  lead  is  more  dense  than  iron,  yet  its  particles 
are  more  easily  separated. 

Caroline,  But  how  are  we  to  judge  of  the  quantity  of  matter 
contained  in  a  certain  bulk  ? 

Mrs.  B.  By  the  weight :  under  the  same  bulk  bodies  are  said 
to  be  dense  in  proportion  as  they  are  heavy. 

Emily.  Then  we  may  say  that  metals  are  dense  bodies,  wood 
comparatively  a  rare  one,  &c.  But,  Mrs.  B.,  when  the  particles 
of  a  body  are  so  near  as  to  attract  each  other,  the  effect  of  this 
power  must  increase  as  they  are  brought  by  it  closer  together ; 
so  that  one  would  suppose  that  the  body  would  gradually  augment 
in  density,  till  it  was  impossible  for  its  particles  to  be  more 
closely  united.  Now,  we  know  that  this  is  not  the  case;  for  soft 
bodies,  such  as  cork,  sponge,  or  butter,  never  become,  in  conse- 
quence of  the  increasing  attraction  of  their  particles,  as  hard  as 
iron  ? 

Mrs.  B.  In  such  bodies  as  cork  and  sponge,  the  particles 
which  come  in  contact  are  so  few  as  to  proauce  but  a  slight  de- 
gree of  cohesion :  they  are  porous  bodies,  which,  owing  to  the 
peculiar  arrangement  of  their  particles,  abound  with  interstices, 
or  pores,  whicn  separate  the  particles.  But  there  is  also  a  fluid 
much  more  subtile  than  air,  which  pervades  all  bodies,  this  is  heat. 
Heat  insinuates  itself  more  or  less  between  the  particles  of  all 
bodies,  and  forces  them  asunder ;  you  may  therefore  consider 
heat,  and  the  attraction  of  cohesion,  as  constantly  acting  in  op- 
position to  each  other. 

Emily.  The  one  endeavouring  to  rend  a  body  to  pieces,  the 
other  to  keep  its  parts  firmly  united. 

Mrs.  B.  And  it  is  this  struggle  between  the  contending  forces 
of  heat  and  ^ittraction,  which  prevents  the  extreme  degree  of 
density  which  would  result  from  the  sole  influence  of  the  attrac- 
tion 01  cohesion. 

Emily.  The  more  a  body  is  heated  then,  the  more  its  parti- 
cles will  be  separated. 

30.  What  is  meant  by  the  term  density?  31.  Do  the  most  dense  bodies 
always  cohere  the  most  strongly  ?  32,  How  do  we  know  that  one  body  is 
more  dense  than  another  ?  33.  What  is  there  which  acts  in  opposition  to  co- 
hesive attraction,  tending  to  separate  the  particles  of  bodies  ? 


GENERAL    PROPERTIES    OF    BODIES.  17 

Mrs,  B.  Certainly :  we  find  that  bodies  not  only  swell  or 
dilate,  but  lose  their  cohesion,  by  heat :  this  eifect  is  very  sensible 
in  butter,  for  instance,  which  expands  by  the  application  of  heat, 
till  at  length  the  attraction  of  cohesion  is  so  far  diminished  that 
the  particles  separate,  and  the  butter  becomes  liquid.  A  similar 
effect  is  produced  by  heat  on  metals?,  and  all  bodies  susceptible 
of  being  melted.  Liquids,  you  know,  are  made  to  boil  by  the 
application  of  heat ;  the  attraction  of  cohesion  then  yields  entirely 
to  the  repulsive  power ;  the  particles  are  totally  separated  and 
convertea  into  steam  or  vapour.  But  the  agency  of  heat  is  in  no 
bodj  more  sensible  than  in  air,  which  dilates  and  contracts  by 
its  increase  or  diminution  in  a  very  remarkable  degree. 

Emily.  The  effects  of  heat  appear  to  be  one  of  the  most  in- 
teresting parts  of  natural  philosophy. 

Mrs.  B.  That  is  true ;  but  heat  is  so  intimately  connected 
with  chemistry,  that  you  must  allow  me  to  defer  the  investiga- 
tion of  its  properties  tdl  you  become  acquainted  with  that  science. 

To  return  to  its  antagonist,  the  attraction  of  cohesion;  it  is  this 
power  which  restores  to  vapour  its  liquid  form,  which  unites  it 
into  drops  when  it  falls  to  earth  in  a  shower  of  rain,  which  gathers 
the  dew  into  brilliant  gems  on  the  blades  of  grass. 

Emily.  And  I  have  often  observed  that  after  a  shower,  tlic 
water  collects  into  large  drops  on  the  leaves  of  plants ;  but  I  can- 
not say  that  I  perfectly  understand  how  the  attraction  of  cohe- 
sion produces  this  effect. 

Mrs,  B.  Rain,  when  it  first  leaves  the  clouds,  is  not  in  the 
form  of  drops,  but  in  that  of  mist  or  vapour,  which  is  composed 
of  very  small  watery  particles ;  these  in  their  descent  mutually 
attract  each  other,  ancl  those  that  are  sufliciently  near  in  conse- 
quence unite  and  form  a  drop,  and  thus  the  mist  is  transformed 
into  a  shower.  The  dew  also  was  originally  in  a  state  of  vapour, 
but  is,  by  the  mutual  attraction  of  the  particles,  formed  into 
small  globules  on  the  blades  of  grass :  in  a  similar  manner  the 
rain  upon  the  leaf  collects  into  large  drops,  which  when  they 
become  too  heavy  for  the  leaf  to  support,  fall  to  the  ground. 

Emily.  All  this  is  wonderfully  curious  !  I  am  almost  bewil- 
dered with  surprise  and  admiration  at  the  number  of  new  ideas 
I  have  already  acquired. 

Mrs.  B.  Every  step  that  you  advance  in  the  pursuit  of  natu- 
ral science,  will  fill  your  mind  with  admiration  and  gratitude 
towards  its  Divine  Author.     In  the  study  of  natural  philosophy, 

34.  What  -would  be  the  consequence  if  the  repulsive  po-^er  of  heat  were  not 
exerted  ?  35.  If  we  continue  to  increase  the  heat,  what  effects  will  it  pro- 
duce on  bodies  ?  36.  What  body  has  its  dimensions  most  sensibly  affected  by- 
change  of  temperature  ?  37.  What  power  restores  vapours  to  the  liquid  form  ? 
38.  What  examples  can  you  give?  39.  JIow  are  d-ops  of  rain  and  of  (^ew 
said  to  be  formed  ? 

B  S 


18  GENERAL    PROPERTIES    OF    BODIES. 

we  must  consider  ourselves  as  reading  the  book  of  nature,  in 
which  the  bountiful  goodness  and  wisdom  of  God  are  revealed 
to  all  mankind ;  no  study  can  tend  more  to  purify  the  heart, 
and  raise  it  to  a  religious  contemplation  of  the  Divine  perfections. 

There  is  another  curious  effect  of  the  attraction  of  cohesion 
which  I  must  point  out  to  you ;  this  is  called  capillary  attraction. 
It  enables  liquids  to  rise  above  their  ordinary  level  in  capillary 
tubes :  these  are  tubes,  the  bores  of  which  are  so  extremely  small 
that  liquids  ascend  within  them,  from  the  cohesive  attraction 
between  the  particles  of  the  liquid  and  the  interior  surface  of 
the  tube.  Do  you  perceive  the  water  rising  in  this  small  glass 
tube,  above  its  level  in  the  goblet  of  water,  into  which  I  nave 
put  one  end  of  it  ? 

Emily.  Oh  yes ;  I  see  it  slowly  creeping  up  the  tube,  but  now 
it  is  stationary :  will  it  rise  no  higher  ? 

Mrs.  B.  No;  because  the  cohesive  attraction  between  the 
water  and  the  internal  surface  of  the  tube  is  now  balanced  by 
the  weight  of  the  water  within  it ;  if  the  bore  of  the  tube  were 
Narrower  the  water  would  rise  higher ;  and  if  you  immerse  seve- 
ral tubes  of  bores  of  different  sizes,  you  will  see  it  rise  to  differ- 
ent heights  in  each  of  them.  In  making  this  experiment,  you 
should  colour  the  water  with  a  little  red  wine,  in  order  to  ren- 
der the  effect  more  obvious. 

All  porous  substances,  such  as  sponge,  bread,  linen,  &c.  may  be 

onsidered  as  collections  of  capillary  tubes:  if  you  dip  one  end  of 

a  lump  of  sugar  into  water,  the  fluid  will  rise  in  it,  and  wet  it 

considerably  above  the  surface  of  the  water  into  which  you  dip  it. 

Emily.  In  making  tea  I  have  often  observed  that  effect,  with- 
tit  being  able  to  account  for  it. 

Mrs.  B.  Now  that  you  are  acquainted  with  the  attraction  of 
ohesion,  I  must  endeavour  to  explain  to  you  that  of  Gravita- 
rion,  which  is  probably  a  modification  of  the  same  power ;  the 
first  is  perceptible  only  in  very  minute  particles,  and  at  very 
small  distances ;  the  other  acts  on  the  largest  bodies,  and  extends 
to  immense  distances. 

Emily.  You  astonish  me :  surely  you  do  not  mean  to  say  that 
lar»e  bodies  attract  each  other  ? 

Mrs.  B.  Indeed  I  do :  let  us  take,  for  example,  one  of  the 
argest  bodies  in  nature,  and  observe  whether  it  does  not  attract 
otlier  bodies.  What  is  it  that  occasions  the  fall  of  this  book, 
wiien  I  no  longer  support  it  ? 

40.  What  is  meant  by  a  capillary  tube  ?  41.  What  eflfect  does  attraction 
produce  when  these  are  immersed  in  water  ?  42.  What  is  the  reason  that 
the  water  rises  to  a  certain  height  only  ?^  43.  Give  some  familiar  examples 
of  capillary  attraction.     44.  In  wliat  does  gravitation  differ  from  cohesive 

t traction?     45.  W^hat  causes  bodies  near  the  earth's  surface,  to  have  a 

;  nd*^ncy  to  fall  towr\i-ds  it? 


GENERAL  PROPERTIES  OF  BODJES.  19 

Emily.  Can  it  be  the  attraction  of  the  earth  ?  I  thought  that 
all  bodies  had  a  natural  tendency  to  fall. 

Mrs.  B.  They  have  a  natural  tendency  to  fall,  it  is  true ;  but 
that  tendency  is  produced  entirely  by  the  attraction  of  the  earth : 
the  earth  being  so  much  larger  than  any  body  on  its  surface, 
forces  every  body,  which  is  not  supported,  to  fall  upon  it. 

Emily.  If  the  tendency  which  bodies  have  to  fall  results 
from  the  earth's  attractive  power,  the  earth  itself  can  have  no 
such  tendency,  since  it  cannot  attract  itself,  and  therefore  it 
requires  no  support  to  prevent  it  from  falling.  Yet  the  idea  that 
bodies  do  not  fall  of  their  own  accord,  but  that  they  are  drawn 
towards  the  earth  by  its  attraction,  is  so  new  and  strange  to  me, 
that  I  know  not  how  to  reconcile  myself  to  it. 

Mrs.  B.  When  you  are  accustomed  to  consider  the  fall  of 
bodies  as  dependin<r  on  this  cause,  it  will  appear  to  you  as  natu- 
ral, and  surely  mucli  more  satisfactory,  than  if  the  cause  of  theii^ 
tendency  to  fall  were  totally  unknown.  Thus  you  understand 
that  all  matter  is  attractive,  from  the  smallest  particle  to  the 
largest  mass;  and  that  bodies  attract  each  other  with  a  force  pro- 
portional to  the  quantity  of  matter  they  contain. 

Emily.  I  do  not  perceive  any  difference  between  the  attrac- 
tion of  cohesion  and  that  of  gravitation ;  is  it  not  because  every 
f)article  of  matter  is  endowed  with  an  attractive  power,  that 
arge  bodies  consisting  of  a  great  number  of  particles,  are  so 
strongly  attractive  ? 

Mrs.  B.  True.  There  is,  however,  tliis  difference  between 
tlie  attraction  of  particles  and  that  of  masses,  that  the  former 
.takes  place  only  when  the  particles  are  contiguous,  whilst  the 
latter  is  exerted  when  the  masses  are  far  from  each  oUier.  The 
attraction  of  particles  frequently  counteracts  the  attraction  of 
gravitation.  Of  this  you  liave  an  instance  in  the  attraction  of 
capillary  tubes,  in  which  liquids  ascend  by  the  attraction  of  cohe- 
sion, in  opposition  to  that  of  gravity.  It  is  on  this  account  that 
it  is  necessary  that  the  bore  of  the  tube  should  be  extremely 
small ;  for  if  the  column  of  water  within  the  tube  is  not  \ery 
minute,  the  attraction  of  cohesion  would  not  be  able  either  to  raise 
or  support  it  in  opposition  to  its  gravity ;  because  the  increase  of 
weight,  in  a  column  of  water  of  a  given  height,  is  much  greater 
than  the  increase  in  the  attracting  surface  of  the  tube,  when  its 
size  is  increased. 

You  may  observe  also,  that  all  solid  bodies  are  enabled  by  the 
force  of  the  cohesive  attraction  of  their  particles  to  resist  that  of 
gravity,  which  would  otherwise  disunite  them,  and  bring  them  to 

46.  What  remarkable  difference  is  there  between  the  attraction  of  gravita- 
tion, and  that  of  cohesion?  47.  In  what  instances  does  the  power  of  cohesion 
counteract  that  of  gravitation?  48.  Why  will  water  rise  to  a  less  height,  if 
the  size  of  the  tube  is  increased  >* 


20  GENERAL   PROPERTIES   OF    BODIES. 

a  level  with  the  ground,  as  it  does  in  the  case  of  a  liquid,  t!ie 
cohesive  attraction  of  which  is  not  sufficient  to  enable  it  to  resist 
the  power  of  gravity. 

Emily,  And  some  solid  bodies  appear  to  be  of  this  nature,  as 
sand,  and  powder  for  instance :  there  is  no  attraction  of  cohesion 
between  their  particles  ? 

Mrs,  B.  Every  grain  of  powder,  or  sand,  is  composed  of  a  great 
number  of  other  more  minute  particles,  firmly  united  by  the  at- 
traction of  cohesion ;  but  amongst  the  separate  grains  there  is  no 
sensible  attraction,  because  they  are  not  m  sufficiently  close  con- 
tact. 

Emily.    Yet  they  actually  touch  each  other  ? 

Mrs.  B,  The  surface  of  bodies  is  in  general  so  rough  and 
uneven,  that  when  in  apparent  contact,  they  touch  each  other  only 
by  a  few  points.  Thus,  when  I  lay  this  book  upon  the  table,  the 
bmding  of  which  appears  perfectly  smooth,  so  few  of  the  par- 
ticles of  its  under  surface  come  in  contact  with  the  table,  that 
no  sensible  degree  of  cohesive  attraction  takes  place ;  for  you  see 
that  it  docs  not  stick  or  cohere  to  the  table,  and  I  find  no  diffi- 
culty in  lifting  it  off. 

It  is  only  when  surfaces,  perfectly  flat  and  well  polished,  are 
placed  in  contact,  that  the  particles  approach  in  sufficient  num- 
ber, and  closely  enough,  to  produce  a  sensible  degree  of  cohesive 
attraction.  Here  are  two  plates  of  polished  metal,  I  press  their" 
flat  surfaces  together,  having  previously  interposed  a  few  drops 
of  oil,  to  fill  up  every  little  porous  vacancy.  Now  try  to  sepa- 
rate them. 

Emily.  It  requires  an  effort  beyond  my  strength,  though 
there  are  handles  for  the  purpose  of  pulling  them  asunder.  Is 
the  firm  adhesion  of  the  two  plates  merely  owing  to  the  attrac- 
tion of  cohesion  ? 

Mrs,  B.  There  is  no  force  more  powerful,  since  it  is  by  this 
lliat  the  particles  of  the  hardest  bodies  are  held  together.  It 
would  require  a  weight  of  several  pounds  to  separate  these  plates. 
In  the  present  example,  however,  much  of  the  cohesive  force  is 
due  to  the  attraction  subsisting  between  the  metal  and  the  oil 
which  is  interposed;  as  ^vithout  this,  or  some  other  fluid,  the 
points  of  contact  would  still  be  comparatively  few,  although  we 
may  have  employed  our  utmost  art,  in  giving  flat  surfaces  to  the 
plates. 

Emily.  In  making  a  kaleidoscope,  I  recollect  that  the  two 
plates  of  glass,  which  were  to  serve  as  mirrors,  stuck  so  fast  to- 
gether, that  I  imagined  some  of  the  gum  I  had  been  using  had 
by  chance  been  interposed  between  them;  but  I  am  now  con- 

49.  Why  do  not  two  bodies  cohere,  when  laid  upon  each  other  ?  50.  Can 
two  bodies  be  made  sufficiently  flat  to  cohere  with  considerable  force? — 
51.  What  is  the  reason  that  the  adhesion  is  greater  when  oil  is  interposed? 


GENERAL   PROPERTIES    OF    BODIES.  21 

vinced  that  it  was  their  own  natural  cohesive  attraction  which 
produced  this  effect. 

Mrs.  B,  Very  probably  it  was  so ;  for  plate -glass  has  an  ex- 
tremely smooth,  flat  surface,  admitting  of  the  contact  of  a  great 
number  of  particles,  when  two  plates  are  laid  upon  each  other. 

Emily,  But,  Mrs.  B.,  the  cohesive  attraction  of  some  sub- 
stances is  much  greater  than  that  of  others ;  thus  glue,  gum  and 
paste,  cohere  witn  singular  tenacity. 

Mrs.  B.  Bodies  which  differ  in  their  natures  in  other  respects, 
differ  also  in  their  cohesive  attraction ;  it  is  probable  that  there 
are  no  two  bodies,  the  particles  of  which  attract  each  other  with 
precisely  the  same  force. 

There  are  some  other  modifications  of  attraction  peculiar  to 
certain  bodies;  namely,  that  of  magnetism,  of  electricity,  and  of 
affinity,  or  chemical  attraction ;  but  we  shall  confine  our  atten- 
tion merely  to  the  attraction  of  cohesion  and  of  gravity ;  the  ex- 
amination of  the  latter  we  shall   resume  at  our  next  meeting. 

52.  What  other  modifications  of  attraction  are  there,  besides  those  of  cohe- 
sion and  of  gi-avitation? 


CONVERSATION  II. 


ON  THE  ATTRACTION  OF  GRAVITY. 

ATTRACTION  OF  GRAVITATION,  CONTINUED. — OF  WEIGHT. — OF  THE  FALL 
OF  BODIES. — OF  THE  RESISTANCE  OF  THE  AIR. — OF  THE  ASCENT  Of 
LIGHT  BODIES. 

EMILY. 

I  HAVE  related  to  my  sister  Caroline  all  that  you  have  taught 
me  of  natural  philosophy,  and  she  has  been  so  much  delighted  by 
it,  that  she  hopes  you  will  have  the  goodness  to  admit  her  to 
your  lessons. 

Mrs.  B.  Very  willingly ;  but  I  did  not  think  you  had  any 
taste  for  studies  of  this  nature,  Caroline. 

Caroline.  I  confess,  Mrs.  B.,  that  hitherto  I  had  formed  no 
very  agreeable  idea  either  of  philosophy,  or  philosophers;  but 
what  Emily  has  told  me  has  excited  my  curiosity  so  much,  that 
1  shall  be  nighly  pleased  if  you  will  allow  me  to  become  one  of 
your  pupils. 

Mrs.  B.  I  fear  that  I  shall  not  find  you  so  tractable  a  scho- 
lar as  Emily;  I  know  that  you  are  much  biased  in  favour  of  your 
own  opinions. 

Caroline.  Then  you  will  have  the  greater  merit  in  reforming 
them,  Mrs.  B.;  and  after  all  the  wonders  that  Emily  has  related 
to  me,  I  think  I  stand  but  little  chance  against  you  and  your 
attractions. 

Mrs.  B.  You  will,  I  doubt  not,  advance  a  number  of  ob- 
jections ;  but  these.  I  shall  willingly  admit,  as  they  will  afford 
an  opportunity  of  elucidatin*>  the  subject.  Emily,  do  you  recol- 
lect tne  names  of  the  general  properties  of  bodies? 

Emily.  Impenetrability,  extension,  figure,  divisibility,  inertia 
and  attraction. 

Mrs.  B.  Very  well.  You  must  remember  that  these  are  pro- 
perties common  to  all  bodies,  and  of  which  they  cannot  be  de- 
prived;  all  other  properties  of  bodies  are  called  accidental,  be- 
cause they  depend  on  the  relation  or  connexion  of  one  body  to 
another. 

1.  What  are  those  properties  of  bodie?  called,  which  are  not  common  to  all? 


ON    THE    ATTRACTION'    OF    GRAVITY.  25 

Caroline.  Yet  surely,  Mrs.  B.  there  are  other  properties  which 
are  essential  to  bodies,  besides  those  you  have  enumerated. 
Colour  and  weight,  for  instance,  are  common  to  all  bodies,  and 
do  not  arise  from  their  connexion  with  each  other,  but  exist  in 
the  bodies  themselves;  these,  therefore,  cannot  be  accidental 
qualities  ? 

Mrs.  B.  I  beg  your  pardon;  these  properties  do  not  exist  in 
bodies  independently  of  their  connexion  with  other  bodies. 

Caroline.  What!  have  bodies  no  weight?  Does  not  tliis  table 
weigh  lieavier  than  this  book ;  and,  if  one  thing  weighs  heavier 
than  another,  must  there  not  be  such  a  thing  as  weight? 

Mrs.  B.  No  doubt :  but  this  property  does  not  appear  to  be 
essential  to  bodies;  it  depends  uiwn  their  connexion  witli  each 
other.  Weight  is  an  eftect  of  the  power  of  attractionj'  without 
which  the  table  and  the  book  would  have  no  weight  whatever. 

Emily.  I  think  I  understand  you;  it  is  the /attraction  of  gra- 
vity which  makes  bodies  heavy. 

Mrs.  B.  You  are  right.  I  told  you  that  the  attraction  of  gra- 
vity was  proportioned  to  the  quantity  of  matter  which  bo  dies  con- 
tain: now  the  earth  consisting  of  a  much  greater  quantity  of 
matter  than  any  body  upon  its  surface,  tlie  force  of  its  attrac- 
tion must  necessarily  be  greatest,  and  must  draw  every  thing 
so  situated  towards  it;  in  consequence  of  whicli,  bodies  that  ai-e 
unsupported  fall  to  the  ground,  whilst  those  that  are  supported, 
press  upon  the  object  wliich  prevents  their  fall,  with  a  weight 
equal  to  the  force  with  which  they  gravitate  towards  the  earth. 

Caroline.  The  same  cause  then  which  occasions  the  fall  of  bo- 
dies, produces  their  weight  also.  It  was  very  dull  in  me  not  to 
understand  this  before,  as  it  is  the  natural  and  necessary  conse- 
quence of  attraction;  but  the  idea  that  bodies  were  not  really 
heavy  of  themselves,  appeared  to  me  quite  incomprehensible. 
But,  Mrs.  B.  if  attraction  is  a  property  essential  to  matter, 
weight  must  be  so  likewise;  for  how  can  one  exist  without  the 
other  ? 

Mrs.  B.  Suppose  there  were  but  one  body  existing  in  univer- 
sal space,  what  would  its  weight  be  ? 

Caroline.  That  would  depend  upon  its  size;  or  more  accu- 
rately speaking,  upon  the  quantity  of  matter  it  contained. 

Emily.  No,  no;  the  body  would  have  no  weight,  whatever 
were  its  size;  because  nothing;  would  attract  it.  Am  I  not  right* 
Mrs.  B.? 

Mrs.  B.  You  are :  you  must  allow,  therefore,  that  it  would  be 
possible  for  attraction  to  exist  without  weight ;  for  each  of  the 

2.  Why  are  they  so  called  ?  3.  What  is  the  cause  of  weight  in  bodies  ? 
4.  What  is  the  reason  that  all  bodies  near  to  the  surface  of  the  e*rth,  are 
drawn  towards  it? 


24  ON   THE    ATTRACTION    OF    GRAVITY. 

particles  of  which  the  body  was  composed,  would  possess  the 
power  of  attraction ;  but  thej  could  exert  it  only  amongst  them- 
selves ;  the  whole  mass  havmg  nothing  to  attract,  or  to  be  at- 
tracted by,  would  have  no  weight. 

Caroline.  I  am  now  well  satisfied  that  weight  is  not  essential 
to  the  existence  of  bodies;  but  what  have  you  to  object  to  co- 
lours, Mrs.  B.;  you  will  not,  I  think,  deny  that  they  really  exist 
in  the  bodies  themselves. 

Mrs.  B.  When  we  come  to  treat  of  the  subject  of  colours,  I 
trust  that  I  shall  be  able  to  convince  you,  that  colours  are  like- 
wise accidental  qualities,  quite  distinct  from  the  bodies  to  which 
they  appear  to  belong. 

Caroline.  Oh  do  pray  explain  it  to  us  now,  I  am  so  very  curi- 
ous to  know  how  that  is  possible. 

Mrs.  B.  Unless  we  proceed  with  some  degree  of  order  and 
method,  you  will  in  the  end  find  yourself  but  little  the  wiser  for 
all  you  learn.  Let  us  therefore  go  on  regularly,  and  make  our- 
selves well  acquainted  with  the  general  properties  of  bodies  be- 
fore we  proceed  further. 

Brnily.  To  return,  then,  to  attraction,  ^which  appears  to  me  by 
far  the  most  interesting  of  them,  since  it  belongs  equa^y  to  all 
kinds  of  matter,)  it  must  be  mutual  between  two  bodies;  and  if 
so,  when  a  stone  falls  to  the  earth,  the  earth  should  rise  part  of 
the  way  to  meet  the  stone? 

Mrs.  B.  Certainly;  but  you  must  recollect  that  the  force  of 
attraction  is  proportioned  to  the  quantity  of  matter  which  bodies 
contain,  and  if  you  consider  the  difference  there  is  in  that  respect, 
between  a  stone  and  the  earth,  you  will  not  be  surprised  that  you 
do  not  perceive  the  earth  rise  to  meet  the  stone;  for  though  it  is 
true  that  a  mutual  attraction  takes  place  between  the  earth  and 
the  stone,  that  of  the  latter  is  so  very  small  in  comparison  to  that 
of  the  former,  as  to  render  its  effect  insensible. 

Emily.  But  since  attraction  is  proportioned  to  the  quantity  of 
matter  which  bodies  contain,  why  do  not  the  hills  attract  the 
houses  and  churches  towards  them  ? 

Caroline.  What  an  idea,  Emily!  How  can  the  houses  and 
churches  be  moved,  when  they  are  so  firmly  fixed  in  the 
ground ! 

Mrs.  B.  Emily's  question  is  not  absuid,  and  your  answer,  Ca- 

5.  If  attraction  is  the  cause  of  weight,  could  you  suppose  it  possible  for  a 
body  to  possess  the  former  and  not  the  latter  property?  6.  When  a  stone  falls 
to  the  ground,  in  which  of  the  two  bodies  does  the  power  of  attraction  exist? 
7.  If  the  attraction  be  mutual,  why  does  not  the  earth  approach  the  stone,  as 
much  as  the  stone  approaches  the  earth  ?  8.  If  attraction  be  in  proportion 
to  the  mass,  why  does  not  a  hill,  draw  towards  itself,  a  house  placed  near 
it? 


ON   THF.    ATTRACTION    OF    GRAVITY.  25 

roline,  is  perfectly  just ;  but  can  you  tell  us  why  the  houses  and 
churches  are  so  firmly  fixed  in  the  ground  ? 

Caroline.  I  am  afraid  I  have  answered  right  by  mere  chance; 
for  I  begin  to  suspect  that  bricklayers  and  carpenters  could  give 
but  little  stability  to  their  buildings,  without  the  aid  of  at- 
traction. 

Mrs.  B.  It  is  certainly  the  cohesive  attraction  between  the 
bricks  and  the  mortar,  which  enables  them  to  build  walls,  and 
these  are  so  strongly  attracted  by  the  earth,  as  to  resist  every 
other  impulse;  otherwise  they  would  necessarily  move  towards 
the  hills  and  the  mountains;  but  the  lesser  force  must  yield  to 
the  greater.  There  are,  however,  some  circumstances  in  which 
I  the  attraction  of  a  large  body  has  sensibly  counteracted  that  of 
^  the  earth.  If  whilst  standing  on  the  declivity  of  a  mountain,  you 
hold  a  plumb-line  in  your  hand,  the  weight  will  not  fall  perpen- 
dicular to  the  earth,  but  incline  a  little  towards  the  mountain ; 
and  this  is  owing  to  the  lateral,  or  sideways  attraction  of  the 
mountain,  interfering  with  the  perpendicular  attraction  of  the 
earth. 

Emily.  But  the  size  of  a  mountain  is  very  trifling,  compared 
to  the  whole  earth. 

Mrs.  B.  Attraction,  you  must  recollect,  is  in  proportion  to  the 
quantity  of  matter,  and  although  that  of  the  mountain,  is  mucl> 
less  than  that  of  the  earth,  it  may  yet  be  sufficient  to  act  sensi- 
bly upon  the  plumb-line  which  is  so  near  to  it. 

Caroline.   Pray  Mrs.  B.  do  the  two  scales  of  a  balance  hang 
•  parallel  to  each  other? 
!        Mrs.  B.  You  mean,  I  suppose,  in  other  words  to  inquire  whe- 
ther two  lines  which  are  perpendicular  to  the  earth,  are  parallel 
to  each  other  ?  I  believe  I  guess  the  reason  of  your  question ;  but 
I  wish  you  would  endeavour  to  answer  it  without  my  assist- 
ance. 
^;        Caroline.   I  was  thinking  that  such  lines  must  both  tend  by 
gravity  to  the  same  point,  the  centre  of  the  earth;  now  lines 
tending  to  the  same  point  cannot  be  parallel,  as  parallel  lines 
are  always  at  an  equal  distance  from  each  other,  and  would 
never  meet. 

Mrs.  B.  Very  well  explained ;  you  see  now  the  use  of  your 
knowledge  of  parallel  lines:  had  you  been  ignorant  of  their  pro- 
perties, you  could  not  have  drawn  such  a  conclusion.     This  may 
'     enable  you  to  form  an  idea  of  the  great  advantage  to  be  derived 
'     even  from  a  slight  knowledge  of  geometry,  in  the  study  of  natu- 
,  ral  philosophy;  and  if  after  I  have  made  you  acquainted  with 

9.  How  can  the  attraction  of  a  mountain  be  rendered  sensible?     10.  Why 
cannot  two  lines  which  are  perpendicular  to  the  surface  of  the  earth  be  pa- 
'(     rallel  to  each  other  ? 

c 


26  ON  THE  ATTRACTION  OF  GRAVITY. 

the  first  elements,  you  should  be  tempted  to  pursue  the  study,  I 
would  advise  you  to  prepare  j^ourselves  by  acquiring  some  know- 
ledge of  geometry.  This  science  would  teach  you  that  lines 
which  fall  perpendicular  to  the  surface  of  a  sphere  cannot  be 
parallel,  because  they  would  all  meet,  if  prolonged  to  the  centre 
of  the  sphere ;  while  lines  that  fall  perpendicular  to  a  plane  or 
flat  surface,  are  always  parallel,  because  if  prolonged,  they  would 
never  meet. 

Emily.  And  yet  a  pair  of  scales,  hanging  perpendicular  to  the 
earth,  appear  parallel  r 

Mrs,  jB.  Because  the  sphere  is  so  large,  and  the  scales  conse- 
quently converge  so  little,  that  their  inclination  is  not  percepti- 
ble to  our  senses ;  if  we  could  construct  a  pair  of  scales  whose 
beam  would  extend  several  degrees,  their  convergence  would  be 
very  obvious ;  but  as  this  cannot  be  accomplished,  let  us  draw  u 
small  figure  of  the  earth,  and  then  we  may  make  a  pair  of  scales 
of  the  proportion  we  please,     (fig.  1.  pi.  I.) 

Caroline.  This  figure  renders  it  very  clear:  then  two  bodies 
cannot  fall  to  the  earth  in  parallel  lines  ? 

Mrs.  B.    Never. 

€aroli7ie.  The  reason  that  a  heavy  body  falls  quicker  than  a 
light  one,  is,  I  suppose,  because  the  earth  attracts  it  more 
{strongly. 

Airs.  B.  The  earth,  it  is  true,  attracts  a  heavy  body  more  than 
a  light  one ;  but  that  would  not  make  the  one  fall  quicker  than 
the  other. 

Caroline.  Yet,  since  it  is  attraction  that  occasions  the  fall  of 
bodies,  surely  the  more  a  body  is  attracted,  the  more  rapidly  it 
will  fall.  Besides,  experience  proves  it  to  be  so.  Do  we  not 
everyday  see  heavy  bodies  fall  quickly,  and  light  bodies  slowly? 

Emily.  It  strikes  me,  as  it  does  Caroline,  that  as  attraction  is 
proportioned  to  the  quantity  of  matter,  the  earth  must  necessa- 
rily attract  a  body  which  contains  a  great  quantity  more  strongly, 
and  therefore  brin^  it  to  the  ground  sooner  than  one  consisting 
of  a  smaller  quantity. 

Mrs.  B.  You  must  consider,  that  if  heavy  bodies  are  attracted 
more  strongly  than  light  ones,  they  require  more  attraction  to 
make  them  fall.  Remember  that  bodies  have  no  natural  ten- 
dency to  fall,  any  more  than  to  rise,  or  to  move  laterally,  and 
that  they  will  not  fall  unless  impelled  hy  some  force;  now  this 
force  must  be  proportioned  to  tlie  quantity  of  matter  it  has  to 
move :  a  body  consisting  of  1000  particles  of  matter,  for  instance, 
requires  ten  times  as  much  attraction  to  bring  it  to  the  ground 
in  the  same  space  of  time  as  a  body  consisting  of  only  100  par- 
ticles. 

11.  Draw  a  small  figure  of  the  earth  to  exemplify  this,  as  in  fig.  1.  plate  1. 


Plate 


J-MJ.  2. 


I'i^.  J. 


# 


ON  THE  ATTRACTION  OF  GRAVITY.  27 

Caroline.  I  do  not  understand  that;  for  it  seems  to  me,  that 
the  heavier  a  body  is,  the  more  easily  and  readily  it  falls. 

Emily.  I  think  I  now  comprehend  it;  let  me  try  if  I  can  ex- 
plain it  to  Caroline.  Suppose  that  I  draw  towards  me  two 
weighty  bodies,  the  one  of  lOOlbs.  the  other  of  lOOOlbs.  must  I 
not  exert  ten  times  as  much  strength  to  draw  the  larger  one  to 
me,  in  the  same  space  of  time,  as  is  required  for  the  smaller  one  r 
And  if  the  eartli  draws  a  body  of  lOOOlbs.  weight  to  it  in  the 
same  space  of  time  that  it  draws  a  body  of  lOOlbs.  does  it  not 
follow  that  it  attracts  the  body  of  lOOOlbs.  weight  with  ten  times 
the  force  that  it  does  that  of  lOOlbs.  ? 

Caroline.  I  comprehend  your  reasoning  perfectly;  but  if  it 
were  so,  the  body  of  lOOOlbs.  weight,  and  that  of  lOOlbs.  would 
fall  with  the  same  rapidity ;  and  the  consequence  would  be,  that 
all  bodies,  whether  light  or  heavy,  being  at  an  equal  distance 
from  the  ground,  would  fall  to  it  in  the  same  space  of  time :  now 
it  is  very  evident  that  this  conclusion  is  absurd;  experience 
every  instant  contradicts  it ;  observe  how  much  sooner  tliis  book 
reaches  the  floor  than  this  sheet  of  paper,  when  I  let  them  drop 
together. 

Emily.  That  is  an  objection  I  cannot  answer.  I  must  refer  it 
to  you,  Mrs.  B.  ^ 

Mrs.  B.  I  trust  that  we  shall  not  find  it  insurmountable.  It 
is  true  that,  according  to  the  laws  of  attraction,  all  bodies  at  an 
equal  distance  from  tne  earth,  should  fall  to  it  in  the  same  space 
of  time;  and  this  would  actually  take  place  if  no  obstacle  inter- 
vened to  impede  their  fall.  But  bodies  fall  through  the  air,  and 
it  is  the  resistance  of  the  air  which  makes  bodies  of  different 
density  fall  with  different  degrees  of  velocity.  They  must  all 
force  their  way  through  the  air,  but  dense  heavy  bodies  over- 
come this  obstacle  more  easily  than  rarer  or  lighter  ones;  be- 
cause in  the  same  space  they  contain  more  gravitating  parti- 
cles. 

The  resistance  which  the  air  opposes  to  the  fall  of  bodies  is 
proportioned  to  their  surface,  not  to  their  weight;  the  air  being 
inert,  cannot  exert  a  greater  force  to  support  the  weight  of  a 
cannon  ball,  than  it  does  to  support  the  weight  of  a  ball  (of  the 
same  size)  made  of  leather;  but  the  cannon  ball  will  overcome 
this  resistance  more  easily,  and  fall  to  the  ground,  consequently, 
quicker  than  the  leather  ball. 

Caroline.  This  is  very  clear  and  solves  the  difficulty  perfectly. 
The  air  offers  the  same  resistance  to  a  bit  of  lead  and  a  bit  of 

12.  If  bodies  were  not  resisted  by  the  air,  those  which  are  light,  would  fall 
as  quickly  as  those  which  are  heavy,  how  can  you  account  for  this  ?  13.  What 
then  is  the  reason  that  a  book,  and  a  sheet  of  paper,  let  fall  from  the  same 
height,  will  act  reach  the  ground  in  tlie  same  time  ' 


28  ON   THE   ATTRACTION   OF    GRAVITY. 

feather  of  the  same  size ;  yet  the  one  seems  to  meet  with  no  ob- 
struction in  its  fall,  whilst  the  other  is  evidently  resisted  and  sup- 
ported for  some  time  by  the  air. 

Emily,  The  larger  the  surface  of  a  body,  then,  the  more  air 
it  covers,  and  the  greater  is  the  resistance  it  meets  with  from  it. 

Airs.  13,  Certainly :  observe  the  manner  in  which  this  sheet 
of  paper  falls ;  it  floats  awhile  in  the  air,  and  then  gently  de- 
scends to  the  ground.  "  I  will  roll  the  same  piece  of  paper  up 
into  a  ball  \  it  offers  now  but  a  small  surface  to  the  air,  and 
encounters  therefore  but  little  resistance :  see  how  much  more 
rapidly  it  falls. 

The  heaviest  bodies  ma^  be  made  to  float  awhile  in  the  air, 
by  making  the  extent  of  their  surface  counterbalance  their  weight. 
Here  is  some  gold,  which  is  one  of  the  most  dense  bodies  we  are 
acquainted  with  ;  but  it  has  been  beaten  into  a  very  thin  leaf,  and 
offers  so  great  an  extent  of  surface  in  proportion  to  its  weight, 
that  its  fall,  you  see,  is  still  more  retarded  by  the  resistance  of 
the  air,  than  that  of  the  sheet  of  paper. 

Caroline.  That  is  very  curious:  and  it  is,  I  suppose,  upon 
the  same  principle  that  a  thin  slate  sinks  in  water  more  slowly 
tiian  a  round  stone. 

But,  Mrs.  B.,  if  the  air  is  a  real  body,  is  it  not  also  subjected 
lo  the  laws  of  gravity.^ 

Mrs.  B,     Undoubtedly. 
'  Caroline.    Then  why  does  it  not,  like  all  other  bodies,  fall  to 
the  ground  ? 

Mrs.  B.  On  account  of  its  spring  or  elasticity.  The  air 
IS  an  elastic  fluid ;  and  the  peculiar  property  of  elastic  bodies 
is  to  resume,  after  compression,  their  original  dimensions;  and 
jou  must  consider  the  air  of  which  the  atmosphere  is  composed 
as  existing  in  a  state  of  compression,  for  its  particles  being 
drawn  towards  the  earth  by  gravity,  are  brought  closer  together 
than  they  would  otherwise  be,  but  the  spring  or  elasticity  of  the 
air  by  which  it  endeavours  to  resist  compression,  gives  it  a  con- 
stant tendency  to  expand  itself,  so  as  to  resume  tne  dimensions 
it  would  naturally  liave,  if  not  under  the  influence  of  gravity. 
The  air  may  therefore  be  said  constantly  to  struggle  with  the 
power  of  gravity  without  being  able  to  overcome  it.  Gravity 
thus  confines  the  air  to  the  regions  of  our  globe,  whilst  its  elasti- 
city prevents  it  from  falling,  like  other  bodies,  to  the  ground. 

Emily.  The  air  then  is,  I  suppose,  thicker,  or  I  should  rather 
say  more  dense,  near  the  surface  of  the  earth,  than  in  the  higher 

14.  What  then  will  be  the  effect  of  mcreasing  the  surface  of  a  body.' — 
13.  What  coulJ  you  do  to  a  sheet  of  paper,  to  make  it  fall  quickly,  and  why  .^ 

16.  Inform  me  how  a  very  dense  body  may  be  made  to  float  in  the  air? — 

17.  The  air  is  a  real  body,  why  does  it  not  fall  to  the  ground  -" 


ON   THE    LAWS    OF    MOTION.  33 

regard  to  motion  or  rest,  it  follows  that  a  body  cannot  move 
without  beinff  put  into  motion ;  the  power  which  puts  a  body  into 
motion  is  called/orce",'  thus  the  stroke  of  the  hammer  is  tlie  force 
which  drives  the  nail ;  the  pulling  of  the  horse  that  which  draws 
the  carriage,  &c.  Force  th^n  is  the  cause  which  produces  motion. 
Emily.  And  may  we  not  say  that  gravity  is  the  force  which 
occasions  the  fall  of  bodies  f 

Mrs.  B.  Undoubtedly.  I  have  given  you  the  most  familiar 
illustrations  in  order  to  render  the  explanation  clear ;  but  since 
you  seek  for  more  scientific  examples,  you  may  say  that  cohesion 
is  the  force  which  binds  the  particles  of  bodies  together,  and  heat 
that  which  drives  them  asunder. 

The  motion  of  a  body  acted  upon  by  a  single  force,  is  always 
in  a  straight  line,'  and  in  the  direction  in  which  it  received  the 
impulse. 

Caroline.  That  is  very  natural ;  for  as  the  body  is  inert,  and 
can  move  only  because  it  is  impelled,  it  will  move  only  in  the 
direction  in  which  it  is  impelled.  The  degree  of  quickness  with 
which  it  moves,  must,  I  suppose,  also  depend  upon  the  degree  of 
force  with  which  it  is  impelled. 

Mrs.  B.  Yes ;  the  rate  at  which  a  body  moves,  or  the  short- 
ness of  the  time  which  it  takes  to  move  from  one  place  to  another, 
is  called  its  velocity ;  and  it  is  one  of  the  laws  of  motion,  that 
the  velocity  of  the  moving  body  is  proportional  to  the  force  by 
which  it  is  put  in  motion.  We  must  distinguish  between  abso- 
lute and  relative  velocity. 

The  velocity  of  a  body  is  called  absolute,  if  we  consider  the 
motion  of  the  body  in  space,  without  any  reference  to  that  of 
other  bodies.  When,  for  instance,  a  horse  goes  fifty  miles  in 
ten  hours,  his  velocity  is  five  miles  an  hour. 

The  velocity  of  a  body  is  termed  relative^  when  compared  with 
that  of  another  body  which  is  itself  in  motion.  For  instance,  if 
one  man  walks  at  the  rate  of  a  mile  an  hour,  and  another  at  the 
rate  of  two  miles  an  hour,  the  relative  velocity  of  the  latter  is 
double  that  of  the  former ;  but  the  absolute  velocity  of  the  one  is 
one  mile,  and  that  of  the  other  two  miles  an  hour. 

Emily.  Let  me  see  if  I  understand  it — The  relative  velocity 
of  a  body  is  the  degree  of  rapidity  of  its  motion  compared  witn 
that  of  another  body ;  thus  ijt  one  ship  sail  three  times  as  far  as 
another  ship  in  thfe  same  space  of  time,  the  velocity  of  the  former 
is  equal  to  three  times  that  of  the  latter. 

3.  What  is  the  consequence  of  inertia,  on  a  body  at  rest  ?  4.  What  do  we 
call  that  which  produces  motion  ?  5.  Give  some  examples.  6.  What  may 
we  say  of  gravity,  of  cohesion,  and  of  heat,  as  forces  ?  7.  How  will  a  body 
move,  if  acted  on  by  a  single  force  ?  8.  What  is  the  reason  of  this  ?  9.  What 
do  we  intend  by  the  term  velocity,  and  to  what  is  it  proportional  ?  10.  Ve- 
locity is  divided  into  absolute  and  relative;  what  is  meant  by  absolute  velo- 
city ?     11.  How  is  relative  velocity  distinguished? 


54  ON   THE   LAWS   OF    MOTION. 

Mrs,  B,  The  general  rule  may  be  expressed  thus :  the  velo- 
city  of  a  body  is  measured  by  the  space  over  which  it  moves, 
divided  by  the  time  which  it  employs  in  that  motion :  thus  if  you 
travel  one  hundred  miles  in  twenty  hours,  what  is  your  velocity 
in  each  hour  ? 

E7mly.  I  must  divide  the  space,  which  is  one  hundred  miles, 
bjr  the  time,  which  is  twenty  hours,  and  the  answer  will  be  five 
miles  an  hour.  Then,  Mrs.  B.,  may  we  not  reverse  this  rule,  and 
say  that  the  time  is  equal  to  the  space  divided  by  the  velocity; 
since  the  space,  one  hundred  miles,  divided  by  the  velocity,  five 
miles  per  hour,  gives  twenty  hours  for  the  time  ? 

Mrs,  B.  Certainly;  and  we  may  say  also  that  the  space  is 
equal  to  the  velocity  multiplied  by  the  time.  Can  you  tell  me, 
Caroline,  how  many  miles  you  will  have  travelled,  if  your  velo- 
city is  three  miles  an  hour,  and  you  travel  six  hours  ? 

Caroline.  Eighteen  miles;  for  the  product  of  3  multiplied  by 
6,  is  18. 

Mrs.  B.  I  suppose  that  you  understand  what  is  meant  by  the 
terms  uniform,  accelerated  and  retarded  motion. 

Emily.  I  conceive  uniform  motion  to  be  that  of  a  body  whose 
motion  is  regular,  and  at  an  equal  rate  throughout ;  for  instance, 
a  horse  that  goes  an  equal  number  of  miles  every  hour.  But  the 
hand  of  a  watch  is  a  much  better  example,  as  its  motion  is  so 
regular  as  to  indicate  the  time. 

Mrs.  B.  You  have  a  right  idea  of  uniform  motion;  but  it 
would  be  more  correctly  expressed  by  saying,  that  the  motion  of 
a  body  is  uniform  when  it  passes  over  equal  spaces  in  equal 
times.  Uniform  motion  is  produced  by  a  force  having  acted  on 
a  body  once  and  having  ceased  to  act ;  as,  for  instance,  the  stroke 
of  a  bat  on  a  ball. 

Caroline.  But  the  motion  of  a  ball  is  not  uniform;  its  velocity 
gradually  diminishes  till  it  falls  to  the  ground. 

Mrs.  JB.  Recollect  that  the  ball  is  inei-t,  and  has  no  more  pow- 
er to  stop,  than  to  put  itself  in  motion ;  if  it  falls,  therefore,  it 
must  be  stopped  by  some  force  superior  to  that  by  which  it  was 
projected,  and  which  destroys  its  motion. 

Caroline.  And  it  is  no  doubt  the  force  of  gravity  which  coun- 
teracts and  destroys  that  of  projection ;  but  ifthere  were  no  such 
power  as  gravity,, would  the  ball  never  stop? 

Mrs.  B.  If  neither  gravity  nor  any  other  force,  such  as  the 
resistance  of  the  air,  opposed  its  motion,  the  ball,  or  even  a  stone 
thrown  by  the  hand,  would  proceed  onwards  in  a  right  line,  and 
with  a  uniform  velocity  for  ever. 

12.  How  "30  we  measure  the  velocity  of  a  body?  13.  The  time?  14.  The 
space  ?  15.  What  is  uniform  motion  ?  and  give  an  example.  16.  How  is 
uniform  motion  produced  ?  17.  A  ball  struck  by  a  bat  gradually  loses  it* 
motion;  what  causes  produce  this  effect i* 


ON   THE    LAWS    OF    MOTION,  35 

Caroline.  You  astonish  me !  I  thought  that  it  was  impossible 
to  produce  perpetual  motion  ? 

Mrs.  B.  Perpetual  motion  cannot  be  produced  by  art,  be- 
cause gravity  ultimately  destroys  all  motion  that  human  power 
can  produce. 

Emily.  But  independently  of  the  force  of  gravity,  I  cannot 
conceive  that  the  little  motion  I  am  capable  of  giving  to  a  stone 
would  put  it  in  motion  for  ever. 

Mrs.  B.  The  quantity  of  motion  you  communicate  to  the 
stone  would  not  influence  its  duration;  if  you  threw  it  with  little 
force  it  would  move  slowly,  for  its  velocity  you  must  r«nember, 
will  be  proportional  to  the  force  with  which  it  is  projected ;  but 
if  there  is  nothin*  to  obstruct  its  passage,  it  will  continue  to  move 
with  the  same  velocity,  and  in  the  same  direction  as  when  you 
first  projected  it. 

Caroline.  This  appears  to  me  quite  incomprehensible ;  we  do 
not  meet  with  a  single  instance  of  it  in  nature. 

Mrs.  B.  I  beg  your  pardon.  When  you  come  to  study  the 
motion  of  the  celestial  bodies,  you  will  find  that  nature  abounds 
with  examples  of  perpetual  motion ;  and  that  it  conduces  as  much 
to  tlie  harmony  of  the  system  of  the  universe,  as  the  prevalence 
of  it  on  the  surface  of  the  earth,  would  to  the  destruction  of 
all  our  comforts.  The  wisdom  of  Providence  has  therefore  or- 
dained insurmountable  obstacles  to  perpetual  motion  here  below; 
and  though  these  obstacles  often  compel  us  to  contend  with  great 
difficulties,  yet  these  appear  necessary  to  that  order,  regularity 
and  repose,  so  essential  to  the  preservation  of  all  the  various 
beings  of  which  this  world  is  composed. 

Now  can  you  tell  me  what  is  retarded  motion  ? 

Caroline.  Retarded  motion  is  that  of  a  body  which  moves  every 
moment  slower  and  slower :  thus  when  I  am  tired  with  walking 
fast,  I  slacken  my  pace ;  or  when  a  stone  is  thrown  upwards,  its 
velocity  is  gradually  diminislied  by  the  power  of  gravity. 

Mrs.  B.  Retarded  motion  is  produced  by  some  force  acting 
upon  the  body  in  a  direction  opposite  to  that  which  first  put  it  in 
motion :  you  who  are  an  animated  being,  endowed  with  power 
and  will,  may  slacken  your  pace,  or  stop  to  rest  when  you  are 
tired ;  but  inert  matter  is  incapable  of  any  feeling  of  fatigue,  can 
never  slacken  its  pace,  and  never  stop,  unless  retarded  or  arrest- 
ed in  its  course  by  some  opposing  force ;  and  as  it  is  the  laws  of 
inert  bodies  of  which  mechanical  philosophy  treats,  I  prefer  your 

18.  If  gravity  did  not  draw  a  projected  body  towards  the  earth,  and  the 
resistance  of  the  air  were  removed,  what  would  be  the  consequence  ?  19.  In 
this  case  would  not  a  great  degree  of  force  be  required  to  produce  a  continued 
motion  ?     20.  What  is  retarded  motion  ?     21 .  Give  some  examples. 


36  ON   THE    LAWS    OF    MOTION. 

t' 

illustration  of  the  stone  retarded  in  its  ascent.  Now  Emily,  it  is 
your  turn;  what  is  accelerated  motion? 

Emily.  Accelerated  motion,  I  suppose,  takes  place  when  the. 
velocity  of  a  body  is  increased ;  if  you  iiad  not  objected  to  our 
giving  such  active  bodies  as  ourselves  as  «xaniples,  I  should  say 
that  my  motion  is  accelerated  if  I  change  my  pace  from  walking 
to  running.  I  cannot  think  of  any  instance  of  accelerated  motion 
in  inanimate  bodies;  all  motion  of  inert  matter  seems  to  be  re- 
tarded by  gravity. 

Mrs.  Ja.  Not  in  all  cases ;  for  the  power  of  gravitation  some- 
times produces  accelerated  motion;  for  instance,  a  stone  falling 
from  a  height,  moves  with  a  regularly  accelerated  motion. 

Emily.  True ;  because  the  nearer  it  approaches  the  earth,  the 
more  it  is  attracted  by  it. 

Mrs.  B.  You  bave  mistaken  the  cause  of  its  accelerated 
motion;  for  though  it  is  true  that  the  force  of  gravity  increases 
as  a  body  approaches  the  earth,  the  difference  is  so  trilling  at  any 
small  distance  from  its  surface,  as  not  to  be  perceptible. 

Accelerated  motion  is  produced  when  the  force  which  put  a 
body  in  motion,  continues  to  act  upon  it  during  its  motion,  so  that 
its  velocity  is  continually  increased.  When  a  stone  falls  from  a 
height,  the  impulse  which  it  receives  from  gravitation  in  the  first 
instant  of  its  fall,  would  be  sufficient  to  bring  it  to  the  ground 
with  a  uniform  velocity:  for,  as  we  have  observed,  a  body  having 
been  once  acted  upon  by  a  force,  will  continue  to  move  with  a 
uniform  velocity;  but  the  stone  is  not  acted  upon  by  gravity 
merely  at  the  first  instant  of  its  fall ;  this  power  continues  to  im- 
pel it  during  the  whole  time  of  its  descent,  and  it  is  this  continu- 
ed impulse  which  accelerates  its  motion. 

Emily.     I  do  not  quite  understand  that. 

Mrs.  B.  Let  us  suppose  that  the  instant  after  you  have  let  a 
stone  fall  from  a  high  tower,  the  force  of  gravity  were  annihilated; 
the  body  would  nevertheless  continue  to  move  downwards,  for 
it  would  have  received  a  first  impulse  from  gravity;  and  a  body 
once  put  in  motion  will  not  stop  unless  it  meets  with  some  ob- 
stacle to  impede  its  course ;  in  this  case  its  velocity  would  be 
uniform,  for  though  there  would  be  no  obstacle  to  obstruct  its 
descent,  there  would  be  no  force  to  accelerate  it. 

Emily.    That  is  very  clear. 

Mrs.  B.  Then  y-ou  have  only  to  add  the  power  of  gravity  con- 
stantly acting  on  tne  stone  during  its  descent,  and  it  will  not  be 
difficult  to  understand  that  its  motion  will  become  accelerated, 
since  the  gravity  which  acts  on  the  stone  at  the  very  first  instant 
of  its  descent,  will  continue  in  force  every  instant,  till  it  reaches 

22.  What  is  accelerated  motion?  23.  Give  an  example.  24.  Explain 
the  mode  in  which  gravity  operates  in  producing  this  effect. 


ON   THE    LAWS    OF    MOTIOiV.  37" 

the  ground.  Let  us  suppose  that  the  impulse  given  by  gravity 
to  the  stone  during  the  first  instant  of  its  descent,  be  equal  to 
one;  the  next  instant  we  shall  find  that  an  additional  impulse 
gives  the  stone  an  additional  velocity,  equal  to  one;  so  that  the 
accumulated  velocity  is  now  equal  to  two ;  the  follovting  instant 
another  impulse  increases  the  velocity  to  three,  and  so  on  till  the 
stone  reaches  the  ground. 

Caroline.  Now  I  understand  it ;  the  effects  of  preceding  im- 
pulses continue,  whilst  gravity  constantly  adds  new  ones,  and 
thus  the  velocity  is  perpetually  increased. 

Mrs.  B,  Yes ;  it  has  been  ascertained,  both  by  experiment,  and 
calculations  which  it  would  be  too  difticult  for  us  to  enter  into, 
that  heavy  bodies  near  the  surface  of  the  earth,  descending  from 
a  height  by  the  force  of  gravity,  fall  sixteen  feet  the  first  second 
of  time,  three  times  that  distance  in  the  next,  five  times  in  the 
tlurd  second,  seven  times  in  the  fourth,  and  so  on,  regularly  in- 
creasing their  velocities  in  the  proportion  of  the  odd  numbers  1, 
3,  5,  7,  9,  &c.  according  to  the  number  of  seconds  during  which 
the  body  has  been  falling. 

Emily.  If  you  throw  a  stone  perpendicularly  upwards,  is  it 
not  the  same  length  of  time  in  ascending,  that  it  is  in  descending  ? 

Mrs.  B.  Exactly;  in  ascending,  the  velocity  is  diminished  by 
the  force  of  gravity;  in  descending,  it  is  accelerated  by  it. 

Caroline.  I  should  then  imagine  that  it  would  fall,  quicker  than 
it  rose? 

Mrs.  B.  You  must  recollect  that  the  force  with  which  it  is  pro- 
jected, must  be  taken  into  the  account ;  and  that  this  force  is 
overcome  and  destroyed  by  gravity,  before  the  bodv  begins  to 
fall. 

Caroline.  But  the  force  of  projection  given  to  a  stone  in  throw- 
ing it  upwards,  cannot  always  be  equal  to  the  force  of  gravity  in 
bringing  it  down  again ;  for  the  force  of  gravity  is  always  the 
same,  whilst  the  degree  of  impulse  ^iven  to  the  stone  is  optional; 
1  may  throw  it  up  gently,  or  with  violence. 

Mrs.  B.  If  you  throw  it  gently,  it  will  not  rise  high ;  perhaps 
only  sixteen  feet,  in  which  case  it  will  fall  in  one  second  of  time. 
Now  it  is  proved  by  experiment,  that  an  impulse  requisite  to 
project  a  body  sixteen  feet  upwards,  will  make  it  ascend  that 
height  in  one  second ;  here  then  the  times  of  the  ascent  and  de- 
scent are  equal.  But  supposing  it  be  required  to  throw  a  stone 
twice  that  height,  the  force  must  be  proportionally  greater. 

You  see  then,  that  the  impulse  of  projection  in  throwing  a  body 
upwards,  is  always  equal  to  the  action  of  the  force  of  gravity 

25.  VV  hat  number  of  feet  will  a  heavy  body  descend  in  the  first  second  ci' 
its  fall,  and  at  what  rate  will  its  velocity  increase  ?  26.  \Miat  is  tlie  differ- 
ence in  the  time  of  the  ascent  and  descent,  of  a  stone,  or  other  body  thrown 
upwards  ?     27.  By  what  reasoning  is  it  proved  that  there  is  no  difference  f 

D 


S8  ON   THE    LAWS    O^    MOTION. 

during  its  descent ;  and  that  whether  the  body  rises  to  a  greater 
or  less  distance,  these  two  forces  balance  each  other. 

I  must  now  explain  to  you  what  is  meant  by  the  momentum 
of  bodies.  It  is  the  force,  or  power,  with  which  a  body  in  mo- 
tion, strikes  against  another  body.  The  momentum  of  a  body  is 
the  product  oi  its  quantity  of  matter,  multiplied  by  its  quantity 
of  motion-,  in  other  words,  its  weight  multiplied  by  its  velocity. 

Caroline.  The  quicker  a  body  moves,  the  greater,  no  doubt, 
must  be  the  force  which  it  would  strike  against  another  body. 

Emily,  Therefore  a  light  body  may  have  a  greater  momen- 
tum than  a  heavier  one,  provided  its  velocity  be  sufficiently  in- 
creased ;  for  instance,  the  momentum  of  an  arrow  shot  from  a 
bow,  must  be  greater  than  that  of  a  stone  thrown  by  the  hand. 

Caroline.  We  know  also  by  experience,  that  the  heavier  a 
body  is,  the  greater  is  its  force ;  it  is  not  therefore  difficult  to 
understand,  that  the  whole  power,  or  momentum  of  a  body,  must 
be  composed  of  these  two  properties,  its  weight  and  its  velocity: 
but  I  do  not  understand  why  they  should  be  multiplied,  the  one 
by  the  other;  I  sliould  have  supposed  that  the  quantity  of  mat- 
ter, should  have  been  added  to  the  quantity  of  motion  ? 

Mrs.  R.  It  is  found  by  experiment,  that  if  the  weight  of  a 
body  is  represented  by  the^number  3,  and  its  velocity  also  by  3, 
its  momentum  will  be  represented  by  9,  not  by  6,  as  would,  be 
the  case,  were  tliese  figures  added,  instead  of  being  multiplied 
together. 

Emily.  I  think  that  I  now  understand  the  reason  of  this ;  if 
tlie  quantity  of  matter  is  increased  three-fold,  it  must  require 
three  times  the  force  to  move  it  with  the  same  velocity;  and  then 
if  we  wish  to  give  it  three  times  the  velocity,  it  will  again  require 
three  times  the  force  to  produce  that  effect,  which  is  three  times 
three,  or  nine;  which  number  therefore,* would  represent  the 
momentum. 

Caroline.  I  am  not  quite  sure  that  I  fully  comprehend  what 
is  intended,  when  weight,  and  velocity,  are  represented  by  num- 
bers alone ;  I  am  so  used  to  measure  space  by  yards  and  miles, 
and  weight  by  pounds  and  ounces,  that  I  still  want  to  associate 
them  together  in  my  mind. 

Mrs.  B.  This  difficulty  will  be  of  very  short  duration :  you 
have  only  to  be  careful,  that  when  you  represent  weights  and 
velocities  by  numbers,  the  denominations  or  values  of  the  weights 
and  spacesj'^must  not  be  changed.  Thus,  if  we  estimate  the  weight 
of  one  body  in  ounces,  the  weight  of  others  with  which  it  is  com- 
pared, must  be  estimated  in  ounces,  and  not  in  pounds :  and  in 

28.  What  is  meant  by  the  momentum  of  a  body  ?  29.  How  do  we  ascertain 
the  momentum  ?  aO.  How  may  a  li^'ht  body  have  a  greater  momentum  than 
one  wliich  is  heavier  ?  31 .  Why  must  we  muUi'ply  the  weight  and  velocity 
together  in  order  to  find  the  momentum  ? 


ON   THE    LAWS    OF    MOTION.  39 

like  manner,  in  comparing  velocities,  we  must  throughQut,  pre- 
serve the  same  standards  both  of  space  and  of  time ;  as  for  in- 
stance, the  number  of  feet  in  one  second,  or  of  miles  in  one  hour. 

Caroline.  I  now  understand  it  perfectly,  and  think  that  I 
shall  never  forget  a  thing  which  you  have  rendered  so  clear. 

Mrs.  B.  I  recommend  it  to  you  to  be  very  careful  to  remem- 
ber the  definition  of  the  momentum  of  bodies,  as  it  is  one  of  the 
most  important  points  in  mechanics :  you  will  find  that  it  is  from 
opposing  velocity,  to  quantity  of  matter,  that  machines  derive 
their  powers. 

The  reaction  of  bodies,  is  the  next  law  of  motion  which  I  must 
explain  to  you.  ^'hen  a  body  in  motion  strikes  against  another 
body,  it  meets  with  resistance  from  it ;  the  resistance  of  the  body 
at  rest  will  be  equal  to  the  blow  struck  by  the  body  in  motion ; 
or  to  express  myself  in  philosophical  language,  action  and  reaction 
will  be  equal,  and  in  opposite  directions. 

Caroline.  Do  you  mean  to  say,  that  the  action  of  the  body 
which  strikes,  is  returned  with  equal  force  by  the  body  which 
receives  the  blow  ? 

Mrs.  B.    Exactly. 

Caroline,  But  if  a  man  strike  another  on  the  face  with  his 
fist,  he  surely  does  not  receive  as  much  pain  by  the  reaction,  as 
he  inflicts  by  the  blow  ? 

Mrs.  B.  No ;  but  this  is  simply  owing  to  the  knuckles,  having 
much  less  feeling  than  the  face. 

Here  are  two  ivory  balls  suspended  by  threads,  (plate  1.  fig. 
3.)  draw  one  of  them.  A,  a  little  on  one  side, — now  let  it  go ; — 
it  strikes,  you  see,  against  the  other  ball  B,  and  drives  it  off,  to 
a  distance  equal  to  that  through  which  the  first  ball  fell ;  but  the 
motion  of  A  is  stopped;  because  when  it  struck  B,  it  received  in 
return  a  blow  equal  to  that  it  gave,  and  its  motion  was  conse- 
quently destroyed. 

Emily.  I  should  have  supposed,  that  the  motion  of  the  ball 
A  was  destroyed,  because  it  had  communicated  all  its  motion  to  B. 

Mrs.  B.  It  is  perfectly  true,  that  when  one  body  strikes 
against  another,  the  quantity  of  motion  communicated  to  the 
second  body,  is  lost  by  the  first ;  but  this  loss  proceeds  from  the 
reaction  of  the  body  which  is  struck. 

Here  are  six  ivory  balls  hanging  in  a  row,  (fig.  4.)  draw  the 
first  out  of  the  perpendicular,  and  let  it  fall  against  the  second. 
You  see  none  of  the  balls  except  the  last,  appear  to  move, 
this  flies  off  as  far  as  the  first  ball  fell;  can  you  explain  this  ? 

32.  When  we  represent  weight  and  velocity  by  numbers,  what  must  we 
carefully  observe  ?  33.  Why  is  it  particularly  important,  to  understand  the 
nature  of  niomentum  ?  34.  What  is  meant  by  reaction,  and  what  is  the  rule 
respecting  it?      35.  How  is  this  exemplified  by  the  ivory  balls  represented 

in  plate  1.  fig.  3.'' 


40  ON   THE    LAWS    OF    MOTIttN, 

Caroline.  I  believe  so.  When  the  first  ball  struck  the  second, 
it  received  a  blow  in  return,  which  destroyed  its  motion;  the 
second  ball,  though  it  did  not  appear  to  move,  must  have  struck 
against  the  third ;  the  reaction  of  which  set  it  at  rest;  the  action 
of  the  third  ball  must  have  been  destroyed  by  the  reaction  of  the 
fourth,  and  so  on  till  motion  was  communicated  to  the  last  ball, 
which,  not  being  reacted  upon,  flies  oif. 

3Ir§.  B.  Very  well  explained.  Observe,  that  it  is  only  when 
bodies  are  elastic,  as  these  ivory  balls  are,  and  when  their  masses 
are  equal,  that  the  stroke  returned  is  equal  to  the  stroke  given,  and 
that  the  striking  body  loses  all  its  motion.  I  will  show  you  the  diifer- 
ence  with  these  two  balls  of  clay,  (fig.  5.)  which  are  not  elastic; 
when  you  raise  one  of  these,  D,  out  of  the  perpendicular,  and  let 
it  fall  against  the  other,  E,  the  reaction  of  the  latter,  on  account 
of  its  not  being  elastic,  is  not  sufficient  to  destroy  the  motion  of 
Jhe  former;  only  part  of  the  motion  of  D  will  be  communicated 
to  E,  and  the  two  balls  will  move  on  togetiier  to  d  and  e,  which 
IS  not  so  great  a  distance  as  that  through  which  D  fell. 

Observe  how  useful  reaction  is  in  nature.  Birds  in  flying 
strike  the  air  with  their  wings,  and  it  is  the  reaction  of  the  air, 
which  enables  them  to  rise,  or  advance  forwards  p  reaction  being 
always  in  a  contrary  direction  to  action. 

Caroline.  I  thought  that  birds  mi^ht  be  lighter  than  the  air, 
when  their  wings  were  expanded,  and  were  by  that  means  ena- 
bled to  fly. 

Mrs.  Jb.  When  their  wings  are  spread,  this  does  not  alter 
their  weight,  but  they  are  better  supported  by  the  air,  as  they 
c€ver  a  greater  extent  of  surface;  yet  they  are  still  much  too 
heavy  to  remain  in  that  situation,  without  continually  flapping 
their  wings,  as  you  may  have  noticed  when  birds  hover  over  their 
nests:  the  force  with  which  their  wings  strike  against  the  air, 
must  equal  the  weight  of  their  bodies,  in  order  that  the  reaction 
of  the  air,  may  be  able  to  support  that  weight;  the  bird  will  then 
lemain  stationary.  If  the  stroke  of  the  wings  is  greater  than  is 
required  merely  to  support  the  bird,  the  reaction  of  the  air  will 
make  it  rise ;  iV  it  be  less,  it  will  gently  descend ;  and  you  may 
have  observed  the  lark,  sometimes  remaining  with  its  wings  ex- 
tended, but  motionless;  in  this  state  it  drops  quietly  into  its 
nest. 

Caroline.  Tliis  is  indeed  a  beautiful  effect  of  the  law  of  reac- 
tion !  But  if  flying  is  merely  a  mechanical  operation,  Mrs.  B., 

36.  Explain  the  manner  in  which  the  six  balls  represented  in  fig.  4,  illus- 
trate thi3  fact.  37.  What  must  be  the  nature  of  bodies,  in  which  the  whole 
motion  is  communicated  from  one  to  the  other  ?  38.  What  is  the  result  if  the 
balls  are  not  elastic,  and  how  is  this  explained  by  fig.  5?  39.  How  will 
reaction  assist  us  in  explaining  the  flight  of  a  binl?  40.  How  must  their 
■\vings  operate  in  enabling  them  to  remain  stationary,  to  rise,  and  to  descend.^ 


0*r  THE   LAWS   OF   MOTION. 


41 


why  should  we  not  construct  wings,  adapted  to  the  size  of  our 
bodies,  fasten  them  to  our  shoulders,  move  them  with  our  arms, 
and  soar  into  the  air  ? 

Mrs,  B,  Such  an  experiment  has  been  repeatedly  attempted, 
but  never  with  success ;  and  it  is  now  considered  as  totally  im- 
practicable. The  muscular  power  of  birds,  i^  incomparably  greater 
in  proportion  to  their  weight,  than  that  of  man ;  were  we  there- 
fore furnished  with  wings  sufficiently  large  to  enable  us  to  fly, 
we  should  not  have  strength  to  put  them  in  motion. 

In  swimming,  a  similar  action  is  produced  on  the  water,  to  that 
on  the  air,  in  flying;  in  rowing,  also,  you  strike  the  water  with 
the  oars,  in  a  direction  opposite  to  that  in  which  the  boat  is  re- 
quired to  move,  and  it  is  the  reaction  of  the  water  on  the  oars 
which  drives  the  boat  along. 

Emily.  You  said,  that  it  was  in  elastic  bodies  only,  that  the 
whole  motion  of  one  body,  would  be  communicated  to  another; 
pray  what  bodies  are  elastic,  besides  the  air  ? 

Mrs.  B.  In  speaking  of  the  air,  I  think  we  defined  elasticity 
to  be  a  property,  by  means  of  which  bodies  that  are  compressed, 
return  to  their  former  state.  If  I  bend  this  cane,  as  soon  as  I 
leave  it  at  liberty,  it  recovers  its  former  position;  if  I  press  my 
finger  upon  your  arm,  as  soon  as  I  remove  it,  the  flesh,  by  virtue 
of  its  elasticity,  rises  and  destroys  the  impression  I  ma3e.  Of 
all  bodies,  the  air  is  the  most  eminent  for  this  property,  and  it 
has  thence  obtained  the  name  of  an  elastic  fluia.  Hard  bodies 
are  in  the  next  degree  elastic ; .  if  two  ivory,  or  hardened  steel 
balls  are  struck  together,  the  parts  at  which  they  touch,  will  be 
flattened;  but  their  elasticity  will  make  them  instantaneously 
resume  their  former  shape. 

Caroline.  But  when  two  ivory  balls  strike  against  each  other, 
as  they  constantly  do  on  a  billiard  table,  no  mark  or  impression 
is  made  by  the  stroke. 

Mrs.  B.  I  beg  your  pardon;  you  cannot, it  is  true,  perceive 
any  mark,  because  their  elasticity  instantly  destroys  all  trace  of  it. 

Soft  bodies,  which  easily  retain  impressions,  such  -  Vclay,  wax, 
tallow,  butter,  &c.  have  very  little  elasticity;  but  of  all  descrip- 
tions of  bodies,  liquids  are  the  least  elastic. 

Emily.  If  sealing-wax  were  elastic,  instead  of  retaining  the 
impression  of  a  seal,  it  would  resume  a  smooth  surface,  as  soon  as 
the  weight  of  tlie  seal  was  removed.  But  pray  what  is  it  that 
produces  the  elasticity  of  bodies? 

Mrs.  B.   There  is  great  diversity  of  opinion  upon  that  point, 

41.  Why  cannot  a  man  fly  by  the  aid  of  wings?  42.  How  does  reaction 
operate  in  enabling  us  to  swim,  or  to  row  a  boat  ?  43.  What  constitute*  elas- 
ticity ?  44.  Give  some  examples.  45.  What  name  is  given  to  air,  and  for 
what  reason?  46.  What  hard  bodies  are  mentioned  as  elastic?  47.  Do 
elastic  bodies  exhibit  any  indentation  after  a  blow?  and  why  not'' 
D  2  ' 


42  ON   THE    LAWS    OF    MOTIO^. 

and  I  cannot  pretend  to  decide  which  approaches  nearest  to  the 
truth.  Elasticity  implies  susceptibility  of  compression,  and  the 
susceptibility  of  compression  depends  upon  the  porosity  of  bo- 
dies ;  for  were  there  no  pores  or  spaces  between  the  particles  of 
matter  of  which  a  body  is  composed,  it  could  not  be  com- 
pressed. 

Caroline.  That  is  to  say,  that  if  the  particles  of  bodies  were 
as  close  together  as  possible,  tliey  could  not  be  squeezed  closer. 

Emily.  Bodies  then,  whose  particles  are  most  distant  from 
each  other,  must  be  most  susceptible  of  compression,  and  conse- 
quently most  elastic ;  and  this  you  say  is  the  case  with  air,  which 
is  perliaps  the  least  dense  of  all  bodies  ? 

Mrs.  B.  You  will  not  in  general  find  this  rule  hold  good  ;  for 
liquids  have  scarcely  any  elasticity,  whilst  hard  bodies  are  emi- 
nent for  this  property,  tnough  the  latter  are  certainly  of  much 
greater  density  than  the  former;  elasticity  implies,  therefore,  not 
only  a  susceptibility  of  compression,  but  depends  upon  the  power 
possessed  by  the  body,  of  resuming  its  former  state  after  com- 
pression, in  consequence  of  the  peculiar  arrangement  of  its  par- 
ticles. 

Caroline.  But  surely  there  can  be  no  pores  in  ivory  and  me- 
tals, Mrs.  B.;  how  then  can  they  be  susceptible  of  compression? 

Mrs.  B.  The  pores  of  such  bodies  are  invisible  to  the  naked 
eye,  but  you  must  not  thence  conclude  that  they  have  none;  it 
is,  on  the  contrary,  well  ascertained  that  gold,  one  of  the  most 
dense  of  all  bodies,  is  extremely  porous ;  and  that  these  pores  are 
sufficiently  large  to  admit  water  when  strongly  compressed,  to 
pass  through  them.  This  was  shown  by  a  celebrated  experiment 
made  many  years  ago  at  Florence. 

Emily.  If  water  can  pass  through  gold,  there  must  certainly 
be  pores  or  interstices  which  afford  it  a  passage ;  and  if  gold  is 
so  porous,  what  must  other  bodies  be,  which  are  so  much  less 
dense  than  gold ! 

Mrs.  B.  The  chief  difference  in  this  respect,  is  I  believe, 
that  the  p<Ji-^s  in  some  bodies  are  larger  than  in  others ;  in  cork, 
sponge  and  bread,  they  form  considerable  cavities;  in  wood  and 
stone,  when  not  polished,  tliey  are  generally  perceptible  to  the 
naked  eye;  whilst  in  ivory,  metals,  and  all  varnisned  and  po- 
lished,  bodies,  they  cannot  be  discerned.  To  give  you  an  idea 
of  the  extreme  porosity  of  bodies,  sir  Isaac  Newton  conjectured 

lat  if  the  earth  were  so  compressed  as  to  be  absolutely  without 
^ores,  its  dimensions  might  possibly  not  be  more  than  a  cubic 
inch. 

48.  What  do  we  conclude  from  elasticity  respecting  the  contact  of  the  parti- 
cles of  a  body  ?     49.  Are  those  bodies  always  the  moat  elastic,  which  are  the 

ast  dense  ?  50.  Give  examples  to  prove  that  this  is  not  tlie  case.  51.  All 
:odies  are  believed  to  be  porous,  what  is  said  on  tlvis  subject  respecting  gold' 


Plate  n. 


ON  THE    LAWS    OF    MOTION  43 

Caroline.  What  an  idea !  Were  we  not  indebted  to  sir  Isaac 
Newton  for  the  theory  of  attraction,  I  should  be  tempted  to  laugh 
at  him  for  such  a  supposition.  What  insignificant  little  crea- 
tures we  should  be ! 

Mrs.  B.  If  our  consequence  arose  from  the  size  of  our  bodies, 
we  should  indeed  be  but  pigmies,  but  remember  that  the  mind  of 
Newton  was  not  circumscribed  by  the  dimensions  of  its  envelope. 

Emily.  It  is,  however,  fortunate  that  heat  keeps  the  pores 
of  matter  open  and  distended,  and  prevents  the  attraction  of  co- 
hesion fix)m  squeezing  us  into  a  nut-shell. 

Mrs.  B.  Let  us  now  return  to  the  subject  of  reaction,  on 
whicli  we  have  some  further  observations  to  make.  It  is  because 
reaction  is  in  its  direction  opposite  to  action,  that  reflected  mo- 
tion is  produced.  If  you  throw  a  ball  against  the  wall,  it  re- 
bounds ;  this  return  of  the  ball  is  owing  to  the  reaction  of  the 
wall  against  which  it  sti'uck,  and  is  called  reflected  motion. 

Emily.  And  I  now  understand  why  balls  filled  with  air  rebound 
better  than  those  stuffed  with  bran  or  wool ;  air  being  most  suscepti- 
ble of  compression  and  most  elastic,  the  reaction  is  more  complete. 

Caroline.  I  have  observed  that  when  I  throw  a  ball  straight 
against  the  wall,  it  returns  straight  to  my  hand ;  but  if  I  throw 
it  obliquely  upwards,  it  rebounds  still  higher,  and  I  catch  it  when 
it  falls. 

Mrs.  B.  You  should  not  say  straight,  but  perpendicularly 
against  the  wall;  for  straight  is  a  general  term  for  lines  in  all 
directions  which  are  neither  curved  nor  bent,  and  is  therefore 
equally  applicable  to  oblique  or  perpendicular  lines. 

Caroline.  I  thought  that  perpendicularly  meant  either  direct- 
ly upwards  or  downwards  ? 

Mrs.  B.  In  those  directions  lines  are  perpendicular  to  the 
earth.  A  perpendicular  line  has  always  a  reference  to  some- 
thing towards  which  it  is  perpendicular;  that  is  to  say,  that  it 
inclines  neither  to  the  one  side  or  the  other,  but  makes  an  equal 
angle  on  every  side.    Do  you  understand  what  an  angle  is  ? 

Caroline.  Yes,  I  believe  so :  it  is  the  space  contained  between 
two  lines  meeting  in  a  point. 

Mrs.  B.  Wen  then,  let  the  line  A  B  (plate  2.  fig.  1.)  repre- 
sent the  floor  of  the  room,  and  the  line  C  D  that  in  which  you 
throw  a  ball  against  it ;  the  line  C  D,  you  will  observe,  forms  two 
angles  with  the  line  A  B,  and  those  two  angles  are  equal. 

Emily.  How  can  the  angles  be  equal,  while  the  lines  which 
compose  them  are  of  unequal  length  ? 

52.  What  conjecture  was  made  by  sir  Isaac  Newton,  respecting  the  porosity 
of  bodies  in  general  ?  53.  If  you  throw  an  elastic  body  against  a  wall,  it  will 
rebound;  what  is  this  occasioned  by,  and  what  is  this  return  motion  called? 
54.  What  do  we  mean  by  a  perpendicular  line?  55.  What  is  an  angle' 
S6.  What  is  represented  by  fig.  1.  plate  2  ? 


44  ON  THB  LAWS   OF  MOTION, 

Mrs.  B.  An  angle  is  not  measured  by  the  length  of  the  lines, 
but  bj  their  opening,  or  the  space  between  them. 

Emily.  Yet  the  longer  the  lines  are,  the  greater  is  the  open- 
ing between  them. 

Mrs.  B.  Take  a  pair  of  compasses  and  draw  a  circle  over 
these  spaces,  making  the  angular  point  the  centre. 

Emily.    To  what  extent  must  I  open  the  compasses? 

Mrs.  B.  You  may  draw  the  circle  what  size  you  please,  pro- 
vided that  it  cuts  the  lines  of  the  angles  we  are  to  measure.  All 
circles,  of  whatever  dimensions,  are  supposed  to  be  divided  into 
360  equal  parts,  called  degrees ;  the  opening  of  an  angle,  being 
therefore  a  portion  of  a  circle,  must  contain  a  certain  number  ot 
degi-ees :  the  larger  the  angle  the  greater  is  the  number  of  degrees, 
and  two  angles  are  said  to  be  equal,  when  they  contain  an  equal 
number  of  degrees. 

Emily.  Now  I  understand  it.  As  the  dimension  of  an  angle 
depends  upon  the  number  of  de<jrees  contained  between  its  lines, 
it  IS  the  opening,  and  not  the  length  of  its  lines,  which  deter- 
mines the  size  oF  the  angle, 

Mrs.  B.  Very  well :  now  that  you  have  a  clear  idea  of  the 
dimensions  of  angles,  can  you  tell  me  how  many  degrees  arc 
contained  in  the  two  angles  formed  by  one  line  falling  perpen- 
dicularly on  another,  as  m  the  figure  1  have  just  drawn  ? 

Emily.  You  must  allow  me  to  put  one  foot  of  the  compasses 
at  the  point  of  the  angles,  and  draw  a  circle  round  them,  and 
then  I  think  I  shall  be  able  to  answer  your  question :  the  two 
angles  are  together  just  equal  to  half  a  circle,  they  contain  there- 
fore 90  degrees  each ;  90  degrees  being  a  quarter  of  360. 

Mrs.  B.  An  angle  of  90  degrees  or  one-fourth  of  a  circle  is 
called  a  right  angle,  and  when  one  line  is  perpendicular  to  an- 
other, and  distant  from  its  ends,  it  forms,  you  see,  (fig.  1.)  a  right 
angle  on  either  side.  Angles  containing  more  tlian  90  degrees 
are  called  obtuse  angles,  (hg.  2.)  and  those  containing  less  than 
90  degrees  are  called  acute  angles,  (fig.  3.) 

Caroline.  The  angles  of  this  square  table  are  right  angles,  but 
^hose  of  the  octagon  table  are  obtuse  angles ;  and  the  angles  of 
sharp  pointed  instruments  are  acute  angles. 

Mrs.  B.    Very  well.    To  return  now  to  your  observation,  that 

57.  Have  the  len^h  of  the  lines  which  meet  in  a  point,  any  thing  to  do 
with  the  measurement  of  an  angle  ?  58.  What  use  can  we  make  of  com- 
passes in  measuring  an  angle  ?  59.  Into  what  number  of  parts  do  we  suppose 
a  whole  circle  divided,  and  what  are  these  parts  called  ?  60.  When  are  two 
angles  said  to  be  equal  ?  61.  Upon  what  does  the  dimension  of  an  angle  de- 
pend ?  62.  What  number  of  degrees,  and  what  portion  of  a  circle  is  there 
in  a  right  angle  ?  63.  How  must  one  line  be  situated  on  another  to  form  two 
right  angles  ?  (fig.  1.)  64.  Figure  2  represents  an  angle  of  more  than  90  de- 
grees, what  is  tliat  called  ?  65.  What  are  those  of  less  than  90  degrees  called 
as  in  fig.  3  .'* 


ON    THE    LAWS    OF    MOTION.  45 

if  a  ball  is  thrown  obliquely  against  the  wall,  it  will  not  rebound 
in  the  same  direction;  tell  me,  have  you  ever  played  at  billiards? 

Caroline.  Yes,  frequently;  and  I  have  observed  that  when  I 
push  the  ball  perpendicularly  against  the  cushion,  it  returns  in 
the  same  direction ;  but  when  I  send  it  obliquely  to  the  cushion, 
'it  rebounds  obliquely,  but  on  an  opposite  side ;  the  ball  in  this 
latter  case  describes  an  angle,  the  point  of  which  is  at  the  cushion. 
I  have  observed  too,  that  the  more  obliquely  the  ball  is  struck 
against  the  cushion,  the  more  obliquely  it  rebounds  on  the  oppo- 
site side,  so  that  a  billiard  player  can  calculate  with  great  accu- 
racy in  what  direction  it  will  return. 

Mrs.  R.  Very  well.  This  figure  (fig.  4.  plate  2.)  represents 
a  billiard  table ;  now  if  you  draw  a  line  A  B  from  the  point 
where  the  ball  A  strikes  perpendicular  to  the  cushion,  you  will 
find  that  it  will  divide  the  angle  which  the  ball  describes  into 
two  parts,  or  two  angles ;  the  one  will  show  the  obliquity  of  the 
direction  of  the  ball  in  its  passage  towards  the  cushion,  the  other 
its  obliquity  in  its  passage  back  from  the  cushion.  The  first  is 
called  the  angle  of  incidence,  the  other  the  angle  of  reflection; 
and  these  angles  are  always  equal,  if  the  bodies  are  perfectly 
elastic. 

Caroline.  This  then  is  the  reason  why,  when  I  throw  a  ball 
obliquely  against  the  wall,  it  rebounds  in  an  opposite  oblique 
direction,  forming  equal  angles  of  incidence  and  of  reflection. 

Mrs.  B.  Certainly ;  and  you  will  find  that  the  more  obliquely 
you  throw  the  ball,  the  more  obliquely  it  will  rebound. 

We  must  now  conclude ;  but  I  shall  have  some  further  obser- 
vations to  make  upon  the  laws  of  motion,  at  our  next  meeting. 

66.  If  you  make  an  elastic  ball  strike  a  body  at  right  angles,  how  will  it  re- 
turn ?  67.  How  if  it  strikes  obliquely  ?  68.  Explain  by  fig.  4  what  is  meant 
by  the  angles  of  incidence  and  of  reflection. 


CONVERSATION  IV. 


ON  COMPOUND  MOTION. 

COBfPOTTiri)  M0TI0I7,  THE  RESULT  OF  TWO  OPPOSITE  FORCES. — OF  CURVI- 
LINEAR MOTION,  THE  RESULT  OF  TWO  FORCES. — CENTRE  OF  MOTION, 
THE  POINT  AT  REST  WHILE  THE  OTHER  PARTS  OF  THE  BODY  MOVE 
ROUND  IT. — CENTRE  OF  MAGNITUDE,  THE  MIDDLE  OF  A  BODY. — CEN- 
TRIPETAL FORCE,  THAT  WHICH  IMPELS  A  BODY  TOWARDS  A  FIXED 
CENTRAL  POINT. — CENTRIFUGAL  FORCE,  THAT  WHICH  IMPELS  A  BODY 
TO  PLY  FROM  THE  CENTRE. — FALL  OF  BODIES  IN  A  PARABOLA. — CEN- 
TRE OF  GRAVITY,  THE  POINT  ABOUT  WHICH  THE  PARTS  BALANCE 
EACH  OTHER. 

MRS.  B. 

I  MUST  now  explain  to  you  the  nature  of  compound  motion. 
Let  us  suppose  a  Dodj  to  be  struck  by  two  equal  forces  in  oppo- 
site directions,  how  will  it  move  ? 

Emily.  If  the  forces  are  equal,  and  their  directions  are  in 
exact  opposition  to  each  other,  I  suppose  the  body  would  not 
move  at  all. 

Mrs.  B.  You  are  perfectl}^  ri^ht;  but  suppose  the  forces 
instead  of  acting  upon  the  body  in  direct  opposition  to  each  other, 
were  to  move  in  lines  forming  an  angle  of  ninety  degrees,  as  the 
lines  YA,  XA,  (fig.  5.  plate  2.)  and  were  to  strike  the  ball  A, 
at  the  same  instant;  would  it  not  move? 

JSrnily.  The  force  X  alone,  would  send  it  towards  B,  and  the 
force  Y  towards  C ;  and  since  these  forces  are  equal,  I  do  not 
know  how  the  body  can  obey  one  impulse  rather  than  the  other; 
and  yet  I  think  the  ball  would  move,  because  as  the  two  forces 
do  not  act  in  direct  opposition,  they  cannot  entirely  destroy  tlie 
effect  of  each  other. 

3frs,  B.  Very  true;  the  ball  therefore  will  not  follow  the  di- 
rection of  either  of  the  forces,  but  will  move  in  a  line  between 
them,  and  will  reach  D  in  the  same  space  of  time,  that  the  force 
X  would  have  sent  it  to  B,  and  the  force  Y  would  have  sent  it 
to  C.  Now  if  you  draw  two  lines,  one  from  B,  parallel  to  A  C, 
and  the  other  from  C,  parallel  to  A  B,  they  will  meet  in  D,  and 

1.  If  a  body  be  struck  by  two  equal  force?  in  opposite  directions,  what  will 
be  the  result  ?     2.  What  is  fig.  5.  plate  2.  intended  to  represent .'' 


ON    COMPOUND    MOTION*  47 

you  will  form  a  square;  the  oblique  line  which  the  body  describes^ 
IS  called  the  diagonal  of  the  square. 

Caroline.  That  is  very  clear,  but  supposing  the  two  forces  to 
be  unequal,  that  the  force  X,  for  instance,  be  twice  as  great  ag 
the  force  Y  ? 

Mrs.  B.  Then  the  force  X,  would  drive  the  ball  twice  as  far 
as  the  force  Y,  consequently  you  must  draw  the  line  A  B  (fig.  6.) 
twice  as  long  as  the  line  A  C,  the  body  will  in  this  case  move  to 
D ;  and  if  you  draw  lines  from  the  points  B  and  C,  exactly  as 
directed  in  the  last  example,  they  will  meet  in  D,  and  you  will 
find  that  the  ball  has  moved  in  the  diagonal  of  a  rectangle. 

Emily.  Allow  me  to  put  another  case.  Suppose  the  two 
forces  are  unequal,  but  do  not  act  on  the  ball  in  the  direction  of 
a  right  angle,  but  in  that  of  an  acute  angle,  what  will  result? 

Mrs.  B.  Prolong  the  lines  in  the  directions  of  the  two  forces, 
and  you  will  soon  discover  which  way  the  ball  will  be  impelled  ;  it 
will  move  from  A  to  D,  in  the  diao;onal  of  a  parallelogram,  (fig.  7.) 
Forces  acting  in  the  direction  of  lines  forming  an  obtuse  angle, 
will  also  produce  motion  in  the  diagonal  of  a  parallelogram.  For 
instance,  if  the  body  set  out  froin  B,  instead  of  A,  and  was  im- 
pelled by  the  forces  X  and  Y,  it  would  move  in  the  dotted  dia- 
gonal B  C. 

We  may  now  proceed  to' curvilinear  motion:  this  is  the  result 
of  two  forces  acting  on  a  body;  by  one  of  which,  it  is  projected 
forward  in  a  right  line;  whilst  by  the  other,  it  is  drawn  or  impel- 
led towards  a  fixed  point.  For  instance,  when  I  whirl  this  ball, 
which  is  fastened  to  my  hand  with  a  string,  the  ball  moves  in  a 
circular  direction,  because  it  is  acted  on  by  two  forces;  that 
which  I  give  it,  which  represents  the  force  of  projection,  and 
that  of  the  string  which  confines  it  to  my  hand.  If,  during  its 
motion  you  were  suddenly  to  cut  the  string,  the  ball  would  fly 
oft*  in  a  straight  line ;  being  released  from  that  confinement  which 
caused  it  to  move  round  a  fixed  point,  it  would  be  acted  on  by 
one  force  only;  and  motion  produced  by  one  force,  you  know,  is 
always  in  a  right  line. 

Caroline.  This  circular  motion,  is  a  little  more  difficult  to 
comprehend  than  compound  motion  in  straight  lines. 

Mrs.  B.  You  have  seen  how  the  water  is  thrown  off  from  a 
grindstone,  when  turned  rapidly  round ;  the  particles  of  the  stone 
itself  have  the  same  tendency,  and  would  also  fly  off,  was  not 
their  attraction  of  cohesion,  greater  than  that  of  water.  And  indeed 

3.  How  would  the  ball  move,  and  how  would  you  represent  the  direction 
of  its  motion?  4.  What  is  supposed  respecting  the  forces  represented  in  fig. 
6?  5.  How  would  the  body  move  if  so  impelled?  6.  If  the  forces  are  une- 
qual and  not  at  right  angles,  how  would  the  body  move,  as  illustrated  by 
fig.  7  ?  7,  How  must  a  body  be  acted  on,  to  produce  motion  in  a  curve, 
and  what  example  'u  given  ? 


48  ON    COMPOUND    WOTION. 

it  sometimes  happens,  that  lai^e  grindstones  flj  to  pieces  from 
the  rapidity  of  their  motion. 

Emily.  In  the  same  way,  the  rim  and  spokes  of  a  wheel, 
when  in  rapid  motion,  would  be  driven  straight  forwards  in  a 
right  line,  were  they  not  confined  to  a  fixed  point,  round  which 
they  are  compelled  to  move. 

Mrs.  B.  Very  well.  You  must  now  learn  to  distinguish  be- 
tween what  is  called  the  centre  of  motion,  and  the  axis  of  motion; 
the  former  being  considered  as  a  point,  the  latter  as  a  line. 

When  a  body,  like  the  ball  at  the  end  of  the  string,  revolves 
in  a  circle,  the  centre  of  the  circle  is  called  the  centre  of  its 
motion,  and  the  body  is  said  to  revolve  in  a  plane;  because  aline 
extended  from  the  revolving  body,  to  the  centre  of  motion,  would 
describe  a  plane,  or  flat  surface. 

When  a  body  revolves  round  itself,  as  a  ball  suspended  by  a 
string,  and  made  to  spin  round,  or  a  top  spinning  on  the  floor, 
whilst  it  remains  on  the  same  spot ;  this  revolution  is  round  an 
imaginary  line  passing  through  the  body,  and  this  line  is  called 
its  axis  of  motion. 

C-aroline.  The  axle  of  a  grindstone,  is  then  the  axis  of  its 
motion ;  but  is  the  centre  of  motion  always  in  the  middle  of  a 
body  ? 

Mrs.  B.  No,  not  always.  The  middle  point  of  a  body,  is 
called  its  centre  of  magnitude,  or  position,  that  is,  the  centre  of 
its  mass  or  bulk.  Bodies  have  also  another  centre,  called  the 
centre  of  gravity,  which  I  shall  explain  to  you ;  but  at  present 
we  must  confine  ourselves  to  the  axis  of  motion.  This  line  you 
must  observe  remains  at  rest,  whilst  all  the  other  parts  of  the 
body  move  around  it;  when  you  spin  a  top,  the  axis  is  stationary, 
whilst  every  other  part  is  in  motion  round  it. 

Caroline.  But  a  top  generally  has  a  motion  forwards  besides 
its  spinning  motion;  and  then  no  point  within  it  can  be  at  rest? 

Mrs.  B.  What  I  say  of  the  axis  of  motion,  relates  only  to 
circular  motion  ;  that  is  to  say,  motion  round  a  line,  and  not  to 
that  which  a  body  may  have  at  the  same  time  in  any  other  di- 
rection. There  is  one  circumstance  to  which  you  must  carefully 
attend;  namely,  that  the  further  any  part  of  a  body  is  from  the 
axis  of  motion,  the  greater  is  its  velocity:  as  you  approach  that 
line,  the  velocity  of  the  parts  gradually  diminish  till  you  reach 
the  axis  of  motion,  which  is  perfectly  at  rest. 

Caroline.     But,  if  every  part  of  the  same  body  did  not  move 

8.  When  is  a  body  said  to  revolve  in  a  plane,  and  what  is  meant  by  the 
centre  of  motion?  9.  What  is  intended  by  the  axis  of  motion,  ad  what  are 
examples?  10.  What  is  the  middle  point  of  a  body  called?  11.  What  is 
said  of  the  axis  of  motion,  whilst  the  body  is  revolving  ?  12.  When  a  body 
revolves  on  an  axis,  do  all  its  parts  move  with  equal  velocity  ? 


Mff.  1. 


'     ^ 


JFlc/  .£> 


OS    COMPOUND    MOTION  4$ 

with  the  same  velocity,  that  part  which  moved  quickest,  must  be 
separated  from  the  rest  of  the  body,  and  leave  it  behind  ? 

Mrs.  B.  You  perplex  yourself  by  confounding  the  idea  of 
circular  motion,  with  that  of  motion  in  a  right  line ;  you  must 
think  only  of  the  motion  of  a  body  round  a  fixed  line,  and  you 
will  find,  that  if  the  parts  farthest  from  the  centre  had  not  the 
greatest  velocity,  those  parts  would  not  be  able  to  keep  up  with 
the  rest  of  the  body,  and  would  be  left  behind.  Do  not  the  ex- 
tremities of  the  vanes  of  a  windmill  move  over  a  much  greater 
space,  than  the  parts  nearest  the  axis  of  motion  ?  (plate  3.  fi».  1.) 
The  three  dotted  circles  represent  the  paths  in  which  three  differ- 
ent parts  of  the  vanes  move,  and  though  the  circles  are  of  differ- 
ent dimensions,  each  of  them  is  described  in  the  same  space 
of  time. 

Caroline.  Certainly  they  are;  and  I  now  only  wonder,  that 
we  neither  of  us  ever  made  the  observation  before:  and  the  same 
effect  must  take  place  in  a  solid  body,  like  the  top  in  spinning; 
the  most  bulging  part  of  the  surface  must  move  with  the  greatest 
rapidity. 

Mrs.  B.  The  force  which  draws  a  body  towards  a  centre*, 
round  which  it  moves,  is  called  the  centripetal  force;  and  that 
force,  which  impels  a  body  to  fly  from  tlie  centre,  is  called  the 
centrifugal  force;  when  a  body  revolves  round  a  centre,  these 
two  mrces  constantly  balance  each  other;  otherwise  the  revolv- 
ing body  would  either  approach  the  centre  or  recede  from  it^ 
according  as  the  one  or  the  other  prevailed. 

Caroline.     When  I  see  any  body  moving  in  a  circle,  I  sha] 
remember,  that  it  is  acted  on  by  two  forces. 

Mrs.  B.     Motion,  either  in  a  circle,  an  ellipsis,  or  any  otlu 
curve-line,  must  be  the  result  of  the  action  of  two  forces;  for 
you  know,  that  the  impulse  of  one  single  force,  always  produces 
motion  in  a  right  line. 

Emily.  And  if  any  cause  should  destroy  the  centripetal  force, 
the  centrifugal  force  would  alone  impel  the  body,  and  it  would, 
I  suppose,  fly  off  in  a  straight  line  from  the  centre  to  which  it 
had  been  confined. 

Mrs.  B.  It  would  not  fly  off  in  a  ri^ht  line  from  the  centre ; 
but  in  a  ri^ht  line  in  the  direction  in  which  it  was  moving,  at  the 
instant  of  its  release ;  if  a  stone,  whirled  round  in  a  sling,  gets 
loose  at  the  point  A,  (plate  3.  fig.  2.)  it  flies  off  in  the  direction 
A  B ;  this  line  is  called  a  tangent,  it  touches  the  circumference 

13.  How  is  this  explained  by  fig.  1.  plate  3  ?  14.  What  are  the  two  forces 
called  which  cause  a  body  to  move  in  a  curve ;  and  what  proportion  do  these 
two  forces  bear  to  each  other  when  a  body  revolves  round  a  centre  ?  15.  If 
the  centripetal  force  were  destroyed,  how  would  a  body  be  carried  by  the 
centrifugal  ? 

E 


50  ON    COMPOUND    MOTION. 

of  the  circle,  and  forms  a  right  angle  with  a  lirie  drawn  from 
that  point  of  the  circumference  to  the  centre  of  the  circle  C. 

Emily.  You  say,  that  motion  In  a  curve -line,  is  owing  to  two 
forces  acting  upon  "a  body;  but  when  I  throw  this  ball  in  a  hori- 
zontal direction,  it  describes  a  curve-line  in  falling;  and  yet  it  is 
only  acted  upon  by  the  force  of  projection;  there  is  no  centripe- 
tal force  to  confine  it,  or  produce  compound  motion. 

Mrs.  B.  A  ball  thus  thrown,  is  acted  upon  by  no  less  than 
three  forces ;  the  force  of  projection,  which  you  communicate  to 
it ;  the  resistance  of  the  air  through  which  it  passes,  which  dimi- 
nishes its  velocity,  without  changmg  its  direction ;  and  the  force 
of  gravity,  which  finally  brings  it  to  the  ground.  The  power  of 
gravity,  and  the  resistance  of  the  air,  being  always  greater  tlian 
any  force  of  projection  we  can  give  a  body,  the  latter  is  gradu- 
ally overcome,  and  the  body  brought  to  the  ground;  but  the 
stronger  the  projectile  force,  the  longer  will  these  powers  be  in 
subduing  it,  and  the  further  the  body  will  go  before  it  falls. 

Caroline.  A  shot  fired  from  a  cannon,  for  instance,  will  go 
much  further,  than  a  stone  projected  by  the  hand. 

Mrs.  B.  Bodies  thus  projected,  you  observe,  describe  a  curve- 
line  in  their  descent;  can  you  account  for  that? 

Caroline.  No ;  I  do  not  understand  why  it  should  not  fall  in 
the  diagonal  of  a  square. 

Mrs.  B.  You  must  consider  that  the  force  of  projection  is 
strongest  when  the  ball  is  first  thrown;  this  force,  as  it  proceeds, 
being  weakened  by  the  continued  resistance  of  the  air,  the  stone, 
therefore,  begins  by  moving  in  a  horizontal  direction ;  but  as  the 
stronger  powers  prevail,  the  direction  of  the  ball  will  gradually 
change  from  a  horizontal,  to  a  perpendicular  line.  Projection 
alone,  would  drive  the  ball  A,  to  B,  (fig.  3.)  gravity  would  bring 
it  to  C ;  therefore,  when  acted  on  in  difterent  directions,  by  these 
two  forces,  it  moves  between,  gradually  inclining  more  and  more 
to  the  force  of  gravity,  in  proportion  as  this  accumulates ;  instead 
therefore  of  reaching  the  ground  at  D,  as  you  suppose  it  would, 
it  falls  somewhere  about  E. 

Caroline.  It  is  precisely  so ;  look  Emily,  as  I  throw  this  ball 
directly  upwards,  how  gravity  and  the  resistance  of  the  air  con- 
quer projection.  Now  I  will  throw  it  upwards  obliquely:  see, 
the  force  of  projection  enables  it,  for  an  instant,  to  act  in  oppo- 
sition to  that  of  gravity ;  but  it  is  soon  brought  down  again. 

Mrs.  B.  The  curve-line  which  the  ball  has  described,  is  call- 
ed in  geometry  a  parabola;  but  when  the  ball  is  thrown  perpen- 
dicularly upwards,  it  will  descend  perpendicularly ;  because  the 

16.  Explain  what  is  meant  by  a  tangent^  as  shown  in  fig.  2.  plate  3. — 
17.  What  forces  impede  a  body  thrown  horizontally  ?  18.  Give  the  reason 
why  a  body  so  projected,  falls  in  a  curve,  (fig.  3.  plate  3.) 


ON    COMPOUND    MOTION.  51 

force  of  projection,  and  that  of  gravity,  are  in  the  same  line  of 
direction.  i    r        •        u 

We  have  noticed  the  centres  of  magnitude,  and  of  motion ;  but 
I  have  nat  jet  explained  to  you,  what  is  meant  by  the  centre  of 
gravity;  it  is  that  point  in  a  body,  about  which  all  the  parts  ex- 
actly balance  each  other;  if  therefore  that  point  be  supported,  the 
bodv  will  not  fall.    Do  you  understand  this  r 

£mily.  I  tliink  so ;  if  the  parts  round  about  this  point  have 
an  equal  tendency  to  fall,  they  will  be  in  equilibrium,  and  as 
long  as  this  point  is  supported,  the  body  cannot  fall. 

Mrs.  B.  Caroline,  what  would  be  the  effect,  were  the  body 
supported  in  any  other  single  point  ? 

Caroline.  The  surrounding  parts  no  longer  balancing  eacli 
other,  the  body,  I  suppose,  would  fall  on  the  side  at  which  tlie 
parts  are  heaviest. 

Mrs.  B.  Infallibly ;  whenever  the  centre  of  gravity  is  unsup- 
ported, the  body  must  fall..  This  sometimes  happens  with  an 
overloaded  wagon  winding  up  a  steep  hill,  one  side  of  the  road 
being  more  elevated  than  the  other ;  let  us  suppose  it  to  slope  as 
is  described  in  this  figure,  (plate  3.  fig.  4.)  we  will  say,  that  the 
centre  of  gravity  of  this  loaded  wagon  is  at  the  point  A.  Now 
your  eye  will  tell  you,  that  a  wagon  thus  situated,  will  overset ; 
and  the  reason  is,  that  the  centre  of  gravity  A,  is  not  supported ; 
for  if  you  draw  a  perpendicular  line  from  it  to  the  ground  at  C, 
it  does  not  fall  under  the  wagon  within  the  wheels,  and  is  there- 
fore not  supported  by  them. 

Caroline.  I  understand  that  perfectly;  but  what  is  the  mean- 
ing of  the  other  point  B  ? 

Mrs.  B.  Let  us,  in  imagination  take  oif  the  upper  part  of 
ihe  load ;  the  centre  of  ^'avity  will  then  change  its  situation,  and 
descend  to  B,  as  that  will  now  be  the  point  about  which  the  parts 
of  the  less  heavily  laden  wagon  will  balance  each  other.  Will 
the  wagon  now  be  upset? 

Caroline.  No,  because  a  perpendicular  line  from  that  point 
falls  within  the  wheels  at  D,  and  is  supported  by  them;  and 
when  the  centre  of  gravity  is  supported,  the  body  will  not  fall. 

Emily.  Yet  I  should  not  much  like  to  pass  a  wagon  in  that 
situation,  for,  as  you  see,  the  npint  D  is  but  just  within  the  left 
wheel ;  ^f  the  riglit  wheel  was  raised,  by  merely  passing  over  a 
stone,  the  point  D  would  be  thrown  on  the  outside  of  the  left 
wheel,  and  the  wagon  would  upset. 

Caroline.    A  wagon,  or  any  carriage  whatever,  will  then  be 

19.  The  curve  in  which  it  falls,  is  not  a  part  of  a  true  circle :  what  is  it  de- 
nominated ?  20.  What  is  the  centre  of  gravity  defined  to  be?  21.  What  re- 
sults from  supporting,  or  not  supporting  the  centre  of  gravity?  22.  What  is 
intended  to  be  explained  by  fig.  4.  plate  3  ?  23.  What  would  be  the  effect  of 
taking  off  the  upper  portion  of  the  load? 


$2  '  ON    CaMPOUND    MOTION. 

most  firmly  supported,  when  the  centre  of  gravity  falls  exactly 
between  the  wheels ;  and  that  is  the  case  in  a  level  road. 

Mrs.  B.  The  centre  of  gravity  of  the  human  body,  is  a  point 
somewhere  in  aline  extending  perpendicularly  through  the  mid- 
dle of  it,  and  as  long  as  we  stand  upright,  this  point  is  supported 
by  the  feet;  if  you  lean  on  one  side,  you  will  find  that  you  no 
longer  stand  firm.  A  rope-dancer  performs  all  his  feats  of  agility, 
by  dexterously  supporting  his  centre  of  gravity;  whenever  he 
finds  that  he  is  in  danger  of  losin»  his  balance,  he  shifts  the  hea- 
vy pole  which  he  holds  in  his  nands,  in  order  to  throw  the 
ueight  towards  the  side  that  is  deficient;  and  thus  by  changing 
Ihe  situation  of  the  centre  of  gravity,  he  restores  his  equilibrnim. 

Caroline.  When  a  stick  is'  poised  on  the  tip  of  the  finger,  is 
It  not  by  supporting  its  centre  of  gravity? 

Airs.  B.  Yes ;  and  it  is  because  the  centre  of  gravity  is  not 
-supported,  that  spherical  bodies  roll  down  a  slope.  A  sphere  be- 
ing perfectly  round,  can  touch  the  slope  but  by  a  single  point, 
aT^d  that  point  cannot  be  perpendicularly  under  the  centre  6f 
^Tavity,  and  therefore  cannot  be  supported,  as  you  will  perceive 
hy  examining  this  figure,  (fig.  5.  plate  3.) 

Emily.  So  it  appears :  yet  I  have  seen  a  cylinder  of  wood 
roll  up  a  slope ;  how  is  that  contrived  ? 

Mrs.  B.  It  is  done  by  plugging  or  loading  one  side  of  the 
cylinder  with  lead,  as  at  B,  (fig.  5.  plate  3.)  the  body  being  no 
longer  of  a  uniform  density,  the  centre  of  gravity  is  removed 
from  the  middle  of  the  body  to  some  point  in  or  near  the  lead, 
as  that  substance  is  much  heavier  than  wood ;  now  you  may  ob- 
serve that  should  this  cylinder  roll  down  the  plane,  as  it  is  here 
situated,  the  centre  of  gravity  must  rise,  which  is  impossible ; 
the  centre  of  gravity  must  always  descend  in  moving,  and  will 
descend  by  the  nearest  and  readiest  means,  which  will  be  by 
forcing  the  cylinder  up  the  slope,  until  the  centre  of  gravity  is 
supported,  and  then  it  stops. 

Caroline.  The  centre  of  gravity,  therefore,  is  not  always  in 
the  middle  of  a  body. 

Mrs.  B.  No,  that  point  we  have  called  the  centre  of  magni- 
tude ;  when  the  body  is  of  an  uniform  density,  and  of  a  regular 
form,  as  a  cube,  or  sphere,  the  c^tres  of  gravity  and  of  magni- 
tude are  in  the  same  point ;  but  when  one  part  of  the  body  isx 
composed  of  heavier  materials  than  another,  the  centre  of  gravity 
can  no  longer  correspond  with  the  centre  of  magnitude.     Thus 

24.  When  will  a  carriage  stand  most  firmly  ?  25.  What  is  said  of  the  cen- 
tre of  gravity  of  the  human  body,  and  how  does  a  rope  dancer  preserve  his 
equilibrium  ?  26.  Why  cannot  a.  sphere  remain  at  rest  on  an  inclined  plane  ? 
(fig.  5.  plate  3.)  27.  A  cylinder  of  wood,  may  be  made  to  rise  to  a  small  dis- 
tance itp  an  inclined  plane.     HoV  may  this  be  effected?  (fig.  5.  plate  3.) 


ON  COMPOUND    MOTION.  53 

you  see  the  centre  of  gravity  of  this  cylinder  plugged  with  lead, 
'  cannot  be  in  the  same  spot  as  the  centre  of  magnitude. 

Emily  Bodies,  therefore,  consisting  but  of  one  kind  of  sub- 
I  stance,  as  wood,  stone,  or  lead,  and  whose  densities  are  conse- 
quently uniform,  must  stand  more  firmly,  and  be  more  difficult  to 
overset,  than  bodies  composed  of  a  variety  of  substances,  of  dif- 
ferent ^nsities,  which  may  throw  the  centre  of  gravity  on  one 
side.      • 

Mrs,  B,  That  depends  upon  the  situation  of  the  materials ; 
if  those  which  are  most  dense,  occupy  the  lower  part,  the  stabili- 
ty will  be  increased,  as  the  centre  of  gravity  will  be  near  the 
base.  But  there  is  another  circumstance  which  more  materially 
affects  the  firmness  of  their  position,  and  that  is  their  form. 
^Bodies  that  have  a  narrow  base  are  easily  upset,  for  if  they  are 
a  little  inclined,  their  centre  of  gravity  is  no  longer  supported, 
as  you  may  perceive  in  fig.  6. 

Caroline,  I  have  often  observed  with  what  difficultya  person 
carries  a  single  pail  of  water ;  it  is  owing,  I  suppose,  to  the  cen- 
tre of  gravity  being  thrown  on  one  side;  and  tlie  opposite  arm  is 
stretched  out  to  endeavour  to  bring  it  back  to  its  original  situa- 
tion; but  a  pail  hanging  to  each  arm  is  carried  with  less  difficulty, 
because  they  balance  each  other,  and  the  centre  of  gravity  re- 
mains supported  by  the  feet. 

Mrs.  B.  Very  well ;  I  have  but  one  more  remark  to  make  on 
the  centre  of  gravity,  which  is,  that  when  two  bodies  are  fastened 
together  by  an  inflexible  rod,  tli*.y  are  to  be  considered  as  form- 
ing but  one  body ;  if  the  two  bodies  be  of  equal  weight,  the  cen- 
tre of  gravitj  will  be  in  the  middle  of  the  line  which  unites  them, 
(fig.  7.)  but  if  one  be  heavier  than  the  other,  the  centre  of  gravity 
will  be  proportionally  nearer  the  heavy  body  than  the  light  one. 
(fig.  8.)  If  you  were  to  carry  a  rod  or  pole  with  an  equal  weight 
fastened  at  each  end  of  it,  you  would  hold  it  in  the  middle  of 
the  rod,  in  order  that  the  weights  should  balance  each  other ; 
whilst  if  the  weights  were  unequal,  you  would  hold  it  nearest 
the  greater  weight,  to  make  them  balance  each  other. 

Emily.  And  in  both  cases  we  should  support  the  centre  of 
gravity;  and  if  one  weight  be  very  considerably  larger  than  the 
other,  the  centre  of  gravity  will  be  thrown  out  of  the  rod  into 
the  heaviest  weight,  (fig.  9.) 

Mrs.  B.    Undoubtedly. 

28.  When  do  we  find  the  centres  of  gravity,  and  of  magnitude  in  different 
points  ?  29.  "What  influence  will  the  density  of  the  parts  of  a  body  exert  up- 
on its  stability  ?  30.  What  other  circumstance  materially  affects  the  firmness 
of  position  ?  (fig.  6.  plate  3.)  31.  Why  is  it  more  easy  to  carry  a  weight  in 
each  hand,  than  in  one  only  ?  32.  What  is  said  respecting  two  bodies  united 
by  an  inflexible  rod?  33.  What  is  fig.  7,  plate  3,  intended  to  iUustrate.' 
Whatfig.  8;whatfig.  9? 
E2 


CONVERSATION  V. 


'  ON  THE  MECHANICAL  POWERS. 

O.B  THE  POWER  OF  MACHINES. — OF  THE  LEVER  IN  GENERAL. — OF  THE 
LEVER  OF  THE  FIRST  KIND,  HAVING  THE  FULCRUM  BETWEEN  THE 
POWER  AND  THE  WEIGHT. — OF  THE  LEVER  OF  THE  SECOND  KIND, 
HAVING  THE  WEIGHT  BETWEEN  THE  POWER  AND  THE  FULCRUM. — 
OF  THE  LEVER  OF  THE  THIRD  KIND,  HAVING  THE  POWER  BETWEEN 
THE  FULCRUM  AND  THE  WEIGHT. 

MRS.  B., 

We  may  now  proceed  to  examine  the  mechanical  powers; 
they  are  six  in  number :  The  lever,  the  pulley,  the  wheel  and  axle, 
the  inclined  plane,  the  wedge  and  the  screw;  one  or  more  of  which 
enters  into  the  composition  of  every  machine. 

A  mechanical  power  is  an  instrument  by  which  the  effect  of 
I  given  force  is  increased,  whilst  the  force  remains  the  same. 

In  order  to  understand  the  power  of  a  machine,  there  are  four 
things  to  be  considered.  1st.  The  power  that  acts:  this  consists 
.in  the  effort  of  men  or  horses,  of  weights,  springs,  steam,  &c. 

2dly.  The  resistance  which  is  to  be  overcome  by  the  power : 
this  is  generally  a  weight  to  be  moved.  Tlie  power  must  always 
be  superior  to  the  resistance,  otherwise  the  machine  could  not  oe 
put  in  motion. 

Caroline,  If  for  instance  the  resistance  of  a  carriage  was 
i^t'eater  than  the  strength  of  the  horses  employed  to  draw  it,  they 
vould  not  be  able  to  make  it  move. 

Mrs.  B.  Sdly.  We  are  to  consider  the  support  or  prop,  or 
ds  it  is  termed  in  mechanics,  the /w/crwm;  this  you  may  recollect 
is  the  point  upon  which  the  body  turns  when  in  motion ;  and 
iastly,  the  respective  velocities  of  the  power,  and  of  the  resist- 
ance; 

Emily.  That  must  in  general  depend  upon  their  respective 
distances  from  the  fulcrum,  or  from  tho  axis  of  motion;  as  we 
observed  in  the  motion  of  the  vanes  of  the  windmill. 

Mrs.  B.     We  shall  now  examine  the  power  of  the  lever.  The 

1 .  How  many  mechanical  powers  are  there,  and  what  are  they  named  ? — 
2.  What  is  a  mechanical  power  defined  to  be  ?  3.  What  four  particulars 
Must  be  observed  ?    4.  Upon  what  will  the  velocities  depend  ^ 


ON   THE    MECHANICAL    POWERS.  55 

/ 

lever  is  an  inflexible  rod  or  bar,  moveable  about  a  fidcrum,  and 
having  forces  applied  to  two  or  more  points  on  it.  For  instance, 
the  steel  rod  to  which  these  scales  are  suspended  is  a  lever,  and 
the  point  in  which  it  is  supported,  the  fulcrum,  or  centre  of  mo- 
tion ;  now,  can  you  tell  me  why  the  two  scales  are  in  equilibrium  ? 

Caroline.  Being  both  empty,  and  of  the  same  weight,  they 
balance  each  other. 

Emily.  Or,  more  correctly  speaking,  because  the  centre  of 
gravity  common  to  both,  is  supported. 

Mrs.  B.  Very  well ;  and  where  is  the  centre  of  gravity  of 
this  pair  of  scales?  (fig.  1.  plate  4.) 

Emily.  You  have  told  us  that  when  two  bodies  of  equal 
weight  were  fastened  together,  the  centre  of  gravity  was  in  the 
middle  of  the  line  that  connected  them;  the  centre  of  gravity  of 
the  scales  must  therefore  be  supported  by  the  fulcrum  F  of  the 
lever  which  unites  the  two  scales,  and  which  is  the  centre  of 
motion. 

Caroline.  But  if  the  scales  contained  different  weights,  the 
centre  of  gravity  would  no  longer  be  in  the  fulcrum  of  the  lever, 
but  remove  towards  that  scale  which  contained  the  heaviest 
weight;  and  since  that  point  would  no  longer  be  supported,  the 
heavy  scale  would  descend,  and  out-weigh  the  other. 

Mrs.  B.  True ;  but  tell  me,  can  you  imagine  any  mode  by 
which  bodies  of  different  weights  can  be  made  to  balance  each 
other,  either  in  a  pair  of  scales,  or  simply  suspended  to  the  ex- 
tremities of  the  lever  ?  for  the  scales  are  not  an  essential  part  of 
the  machine ;  they  have  no  mechanical  power,  and  are  used  merely 
for  the  convenience  of  containing  the  substance  to  be  weighed. 

Caroline.  What!  make  a  light  body  balance  a  heavy  one ?  I 
cannot  conceive  that  possible. 

Mrs.  B.  The  fulcrum  of  this  pair  of  scales  (fig.  2.)  is  movea- 
ble, you  see ;  I  can  take  it  off  the  beam,  and  fasten  it  on  again  in 
another  part;  this  part  is  now  become  the  fulcrum,  but  it  is  no 
longer  in  the  centre  of  the  lever. 

Caroline.  And  the  scales  are  no  longer  true ;  for  that  which 
hangs  on  the  longest  side  of  the  lever  descends. 

Mrs.  B.  The  two  parts  of  the  lever  divided  by  the  fulcrum, 
are  called  its  arms ;  you  should  therefore  say  the  longest  arm, 
not  the  longest  side  of  the  lever. 

Your  observation  is  true  that  the  balance  is  now  destroyed ; 
but  it  will  answer  the  purpose  of  enabling  you  to  comprehend 
the  power  of  a  lever,  when. the  fulcrum  is  not  in  the  centre. 

5.  What  is  a  lever  ?  6.  Give  a  familiar  example.  7.  When  and  why  do 
the  scales  balance  each  other,  and  where  is  their  centre  of  gravity?  (fig.  1. 
plate  4.)  8.  Why  would  they  not  balance  with  unequal  weights  ?  9.  Were 
the  fulcrum  removed  from  the  middle  of  the  beam  %bat  would  result? 
fO.  What  do  we  mean  by  the  arma  of  a  lever  ? 


56  ON  THE   MECHANICAL   POWERS. 

Emily,  This  would  be  an  excellent  contrivance  for  those  who 
cheat  in  the  weight  of  their  goods ;  by  making  the  fulcrum  a  lit- 
tle on  one  side,  and  placing  the  goods  in  the  scale  which  is  sus- 
pended to  the  longest  arm  of  the  lever,  they  would  appear  to 
weigh  more  than  they  do  in  reality. 

Mrs,  B.  You  do  not  consider  how  easily  the  fraud  would  be 
detected  ;  for  on  the  scales  bein*  emptied  they  would  not  hang 
in  equilibrium.  If  indeed  the  scale  on  the  shorter  arm  was  made 
heavier,  so  as  to  balance  that  on  the  longer,  they  would  appear 
to  be  true,  whilst  they  were  really  false.} 

Emily.  True;  I  did  not  think  of  that  circumstance.  But  I 
do  not  understand  why  the  longest  arm  of  the  lever  should  not 
be  in  equilibrium  with  the  other  } 

Caroline.  It  is  because  the  momentum  in  the  longest,  is  greater 
than  in  the  shortest  arm;  the  centre  of  gravity,  tlierefore,  is  no 
longer  supported. 

Mrs.  B.  You  are  right,  the  fulcrum  is  no  longer  in  the  cen- 
tre of  gravity ;  but  if  we  can  contrive  to  make  the  fulcrum  in  its 
present  situation  become  the  centre  of  gravity,  the  scales  will 
again  balance  each  other ;  for  you  recollect  that  the  centre  of 
gi-avity  is  that  point  about  which  every  part  of  the  body  is  in 
equilibrium. 

Emily.  It  has  just  occurred  to  me  how  this  may  be  acoom- 
plished;  put  a  great  weight  into  the  scale  suspended  to  the 
shortest  arm  of  tlie  lever,  and  a  smaller  one  into  that  suspended 
to  the  longest  arm.  Yes,  I  have  discovered  it — look  Mrs.  B., 
the  scale  on  the  shortest  arm  will  carry  3lbs.,  and  that  on  the 
longest  arm  only  one,  to  restore  the  balance,  (fig.  3.) 

Mrs.  B.  You  see,  therefore,  that  it  is  not  so  impracticable 
as  you  imagined,  to  make  a  heavy  body  balance  a  light  one;  and 
this  is  in  fact  the  means  by  which  you  observed  that  an  imposi- 
tion in  the  weight  of  goods  might  be  effected,  as  a  weight  oi  ten 
or  twelve  ounces,  might  thus  be  made  to  balance  a  pound  of 
goods.  If  you  measure  both  arms  of  the  lever,  you  will  find  that 
the  length  of  the  longer  arm,  is  three  times  that  of  the  shorter; 
and  that  to  produce  an  equilibrium,  the  weights  must  bear  the 
same  proportion  to  each  other,  and  that  the  greater  weight,  must 
be  on  the  sliorter  arm.  Let  us  now  take  oil  the  scales,  that  we 
may  consider  the  lever  simply;  and  in  this  state  you  see  that  the 
fulcrum  is  no  longer  the  centre  of  gravity,  because  it  has  been 
removed  from  the  middle  of  the  lever ;  but  it  is,  and  must  ever 
be,  the  centre  of  motion,  as  it  is  the  only  point  which  remains  at 
rest,  while  the  other  parts  move  about  it. 

11.  How  may  a  pair  of  scales  be  false,  and  yet  appear  to  be  true  ?  12.  If 
the  fulcrum  be  removed  from  the  centre  of  gravity,  how  may  the  equilibrium 
be  restored  ?  13.  How  is  this  exemplified  by  fig.  3.  plate  4  i*  14>  What  pro- 
portion must  the  weights  bear  to  the  lengths  of  the  arms  ? 


ON   THE    MECHANICAL    POWERS.  ^7 

Caroline.  The  arms  of  the  lever  being  different  in  length,  it 
now  exactly  resembles  the  steelyards,  with  which  articles  are 
so  frequently  weighed. 

Mrs.  B.  It  may  in  fact  be  considered  as  a  pair  of  steelyards, 
by  which  J^he  same  power  enables  us  to  ascertain  the  weight  of 
different  articles,  by  simply  increasing  the  distance  of  the  power 
from  the  fulcrum ;  you  know  that  the  farther  a  body  is  from  the 
axis  of  motion,  the  greater  is  its  velocity. 

Caroline,     That  I  remember,  and  understand  perfectly. 

Mrs.  B.  You  comprehend  then,  that  the  extremity  of  the 
lon;jest  arm  of  a  lever,  must  move  with  greater  velocity  than  that 
of  tlie  shortest  arm,  and  that  its  momientum  is  greater  in  propor- 
tion. 

Emily.  No  doubt,  because  it^is  farthest  from  the  centre  of 
motion.  And  pray,  Mrs.  B.,  when  my  brothers  play  at  see-saWj 
is  not  the  plank  on  which  they  ride,  a  kind  of  lever  ? 

Mrs.  B.  Certainly ;  the  log  of  wood  which  supports  it  from 
the  ground  is  the  fulcrum,  and  those  who  ride,  represent  the 
power  and  the  resistance  at  the  ends  of  the  lever.  And  have  you 
not  observed  that  when  those  who  ride  are  of  equal  weight,  the 
plank  must  be  supported  in  the  middle,  to  make  the  two  arms 
equal ;  whilst  if  the  persons  differ  in  weight,  the  plank  must  be 
drawn  a  little  ferther  over  the  prop,  to  make  the  arms  unequal, 
and  the  lightest  person,  who  may  be  supposed  to  represent  the 
power,  must  be  placed  at  the  extremity  of  the  longest  arm. 

Caroline.  That  is  always  the  case  when  I  ride  on  a  plank 
with  my  youngest  brother ;  I  have  observed  also  that  the  lightest 
person  has  the  best  ride,  as  he  moves  both  further  and  quicker ; 
and  I  now  understand  that  it  is  because  he  is  more  distant  from 
the  centre  of  motion. 

Mrs.  B.  The  greater  velocity  with  which  your  little  bi'other 
moves,  renders  his  momentum  equal  to  yours. 

Caroline.  Yes ;  I  have  the  most  weight,  he  the  greatest  velo- 
city; so  that  upon  the  whole  our  momentums  are  equal.  But 
you  said,  Mrs.  B.,  that  the  power  should  be  greater  than  the  re- 
sistance, to  put  the  machine  in  motion ;  how  then  can  the  plank 
move  if  the  momentums  of  the  persons  who  ride  are  equal? 

Mrs.  B.  Because  each  person  at  his  descent  touches  and 
pushes  against  the  ground  with  his  feet ;  the  reaction  of  which 
gives  him  an  impulse  which  produces  the  motion  ;  this  spring  is 
requisite  to  destroy  the  equilibrium  of  the  power  and  the  resist- 
ance, otherwise  the  plank  would  not  move.  Did  you  ever  ob- 
serve that  a  lever  describes  the  arc  of  a  circle  in  its  motion  ? 

15.  On  what  principle  do  we  Weigh  with  a  pair  of  steelyards,  and  what 
will  be  the  difference  in  the  motion  pf  the  extremities  of  euch  a  lever  ? 


5$  ON   THE    MECHANICAL    POWERS. 

Emily.  No;  it  appears  to  me  to  rise  and  descend  perpendi- 
cularly ;  at  least  I  always  thought  so. 

Mrs.  B.  I  believe  I  must  make  a  sketch  of  you  and  your 
brother  riding  on  a  plank,  in  order  to  convince  you  of  your  error, 
(fie.  4.  plate  4.)  You  may  now  observe  that  a  lever  can  move 
only  round  the  fulcrum, 'since  that  is  the  centre  of  motion ;  it 
would  be  impossible  for  you  to  rise  pei-pendicularly,  to  the  point 
A;  or  for  your  brother  to  descend  in  a  straight  line,  to  the  point 
B ;  you  must  in  rising,  and  he  in  descending,  describe  arcs  of  your 
respective  circles.  This  drawing  shows  you  also  how  much  su- 
perior his  velocity  must  be  to  yours ;  for  if  you  could  swing  quite 
round,  you  would  each  complete  your  respective  circles,  in  the 
same  time. 

Caroline.  My  brother's  circle  being  much  the  largest,  he 
must  undoubtedly  move  the  quickest. 

Mrs.  B.  Now  tell  me,  do  you  think  that  your  brother  could 
raise  you  as  easily  without  the  aid  of  a  lever  ? 

Caroline.    Oh  no,  he  could  not  lift  me  off  the  ground. 

Mrs.  B.  Then  I  think  you  require  no  further  proof  of  the 
power  of  a  lever,  since  you  see  what  it  enables  your  brother  to 
perform. 

Caroline.  I  now  understand  what  you  meant  by  saying,  that 
in  mechanics,  velocity  is  opposed  to  weight,  for  it  is  my  brother's 
velocity  which  overcomes  my  weight. 

Mrs.  B.  You  may  easily  imagine,  what  enormous  weights 
may  be  raised  by  levers  of  this  description,  for  the  longer,  wnen 
compared  with  the  other,  that  arm  is  to  which  the  power  is  ap- 
plied, the  greater  will  be  the  effect  produced  by  it;  because  the 
greater  is  the  velocity  of  the  power  compared  to  that  uf  the  weight. 

Levers  are  of  three  kinds ;  in  the  first  the  fulcrum  is  between 
the  power  and  the  weight. 

Caroline.  This  kind  then  comprehends  the  several  levers  you 
have  described. 

Mrs.  B.  Yes,  when  in  levers  of  the  first  kind,  the  fulcrum  is 
equally  distant  from  the  power  and  the  weight,  as  in  the  balance, 
there  will  be  an  equilibrium,  when  the  power  and  the  weight 
are  equal  to  each  other;  it  is  not  then  a  mechanical  power,  for 
nothing  can  in  this  case  be  gained  by  velocity;  the  two  aims  of 
the  lever  being  equal,  the  velocity  of  their  extremities  must  be 
so  likewise.  The  balance  is  therefore  of  no  assistance  as  a  me- 
chanical power,  although  it  is  extremely  useful  in  estimating  the 
respective  weights  of  bodies. 

But  when  (fig.  5.)  the  fulcrum  F  of  a  lever  is  not  equally  dis- 

16.  How  is  this  exemplified  by  fi^.  4.  plate  4?  17.  What  line  is  descri- 
bed by  the  ends  of  a  lever  ?  fig.  4.  plate  4.  18.  How  many  kinds  are  there; 
and  in  the  first  how  is  the  fulcrum  situated?  19.  When  may  the  fulcrum  be 
se  situated  that  this  lever  ia  not  a  mechanical  power,  and  why  ? 


ON   THE    MECHANICAL    POWERS-  59 

tant  from  the  power  and  the  weight,  and  the  power  P  acts  at  the 
extremity  of  the  longest  arm,  it  may  be  less  tnan  the  weight  W ; 
its  deficiency  being  compensated  by  its  superior  velocity,  as  we 
obser\'ed  in  the  see-saw.' 

Emily,  Then  when  we  want  to  lift  a  great  weight,  we  must 
fasten  it  to  the  shortest  arm  of  a  lever,  and  apply  our  strength  to 
t!ie  longest  arm  ? 

Mrs.  B.  If  the  case  will  admit  of  your  putting  the  end  of  the. 
lever  under  the  resisting  body,  no  fastening  will  be  required;  as 
you  will  perceive,  when  a  nail  is  drawn  by  means  of  a  hammer, 
which,  though  bent,  is  a  lever  of  the  first  liind;  the  handle  being 
the  longest  arm,  the  point  on  which  it  rests,  the  fulcrum,  and  the 
distance  from  that  to  the  part  which  holds  the  nail,  the  short  arm. 
But  let  me  hear,  Caroline,  whether  you  can  explain  the  action 
of  this  instrument,  which  is  composed  of  two  levers  united  in  one 
common  fulcrum. 

Caroline.     A  pair  of  scissors  ! 

Mrs.  B,  You  are  surprised;  but  if  you  examine  their  con- 
struction, you  will  discover  tliat  it  is  the  power  of  tlie  lever,  that 
assists  us  in  cutting  with  scissors. 

Caroline.  Yes ;  I  now  perceive  that  the  point  at  which  the 
two  levers  are  screwed  together,  is  the  fulcrum ;  the  power  of 
the  fingers  is  applied  to  the  handles,  and  the  article  to  be  cut,  is 
the  resistance ;  therefore,  the  longer  the  handles,  and  the  shorter 
the  points  of  the  scissors,  the  more  easily  you  cut  with  them. 

Emily.  That  I  have  often  observed,  for  when  I  cut  paste- 
board or  any  hard  substance,  I  always  make  use  of  that  part  of 
the  scissors  nearest  the  screw  or  rivet,  and  I  now  understand 
M'hy  it  increases  the  power  of  cutting;  but  I  confess  that  I  never 
should  have  discovered  scissors  to  have  been  double  levers;  and 
pray  are  not  snuffers  levers  of  a  similar  description  ? 

Airs.  B.  Yes,  and  most  kinds  of  pincers ;  the  great  power  of 
which  consists  in  the  great  relative  length  of  the  handles. 

Did  you  ever  notice  the  swingle-tree  of  a  carriage  to  which 
the  horses  are  attached  when  drawing  ? 

Emily.     O  yes ;  this  is  a  lever  of  the  first  kind,  but  the  ful 
crum  being  in  the  middle,  the  horses  should  draw  with  equal 
power,  whatever  may  be  their  strength. 

Mrs.  B.  That  is  generally  the  case,  but  it  is  evident  that  by 
making  one  arm  longer  than  the  other,  it  might  be  adapted  to 
horses  of  unequal  strength. 

Caroline.  And  of  what  nature  are  the  other  two  kinds  of  le- 
vers ? 

20.  What  is  represented  by  fig.  5.  plate  4?  21.  Give  a  familiar  example 
of  the  use  of  a  lever  of  the  first  kind.  22.  In  what  instruments  are  two  such 
levers  combined  ?  23.  How  may  two  horses  of  unequal  strength,  be  advan- 
tageously coupled  in  a  carriage  f 


©O  ON    THE    MECHANICAL    POWERS. 

Mrs,  B,  In  levers  of  the  second  kind,  the  weight,  instead  of 
being  at  one  end,  is  situated  between  the  power  and  the  ful- 
crum, (fig.  6.) 

Caroline.  The  weight  and  the  fulcrum  have  here  changed 
places ;  and  what  advantage  is  gained  bj  this  kind  of  lever  ? 

Mrs,  B,  In  moving  it,  tlie  velocity  of  the  power  must  neces- 
sarily be  greater  than  that  of  the  weight,  as  it  is  more  distant 
from  the  centre  of  the  motion.  Have  you  ever  seen  your  brother 
move  a  snow-ball  by  means  of  a  strong  stick,  when  it  became  too 
heavy  for  him  to  move  without  assistance  ? 

Caroline.  Oh  yes ;  and  this  was  a  lever  of  the  second  kind, 
(fi^.  7.)  the  end  of  the  stick,  which  he  thrusts  under  the  ball,  and 
which  rests  on  the  ground,  becomes  the  fulcrum  ;  the  ball  is  the 
weight  to  be  moved,  and  the  power  his  hands,  applied  to  the 
other  end  of  the  lever.  In  this  instance  th«re  is  a  great  differ- 
ence in  the  length  of  the  arms  of  the  lever ;  for  the  weight  is  al- 
most close  to  the  fulcrum. 

Mrs.  B.  And  the  advantage  gained  is  proportional  to  this 
difference.  The  most  common  example  that  we  have  of  levers 
of  the  second  kind,  is  in  the  doors  of  our  apartments. 

Emily.  The  hinges  represent  the  fulcrum,  our  hands  the 
power  applied  to  the  other  end  of  the  lever  j  but  where  is  the 
weight  to  be  moved  ?  ^ 

Mrs,  B.  The  door  is  the  weight,  which  in  this  example  occu- 
pies the  whole  of  the  space  between  the  power  and  the  fulcrum. 
Nut  crackers  are  double  levers  of  this  kind  :  the  hinge  is  the  ful- 
crum, the  nut  the  resistance,  and  the  hands  the  power. 

In  levers  of  the  third  kind  (fig.  8.)  the  fulcrum  is  again  at  one 
extremity,  the  weight  or  resistance  at  the  other,  and  the  power  is 
applied  between  the  fulcrum  and  the  resistance. 

Emily.  The  fulcrum,  the  weight,  or  the  power,  then,  each 
in  its  turn,  occupies  some  part  of  the  lever  between  its  extremi- 
ties. But  in  this  third  kind  of  lever,  the  weight  being  farther 
than  the  power  from  the  centre  of  motion,  the  difficulty  of  rais- 
ing it  seems  increased  rather  than  diminished. 

Mrs.  B,  That  is  very  true ;  a  lever  of  this  kind  is  therefore 
never  used,  unless  absolutely  necessary,  as  is  the  case  in  raising 
a  ladder  in  order  to  place  it  against  a  wall  J  the  man  who  raises 
it  cannot  place  his  nands  on  the  upper  part  of  the  ladder^  the 
power,  therefore,  is  necessarily  placed  much  nearer  to  the  ful- 
crum than  to  the  weight. 

Caroline,  Yes,  the  hands  are  the  power,  the  ground  the  ful- 
crum, and  the  upper  part  of  the  ladder  the  weight. 

24.  DescribiR  a  lever  of  the  second  kind.  (Fig.  6.  plate  4.)  25.  What  is 
represented  in  fig.  7.  plate  4,  and  in  what  proportion  does  this  lever  gain 
power?  26.  What  is  said  respecting  a  door?  27.  Describe  a  lever  of  the 
third  kind.    28.  In  what  instance  do  we  use  this  ? 


«N   THE    MECHAIflCAL    POWERS.  61 

Mrs,  B.  Nature  employs  this  kind  of  lever  in  the  structure 
of  the  human  frame.  In  lifting  a  weight  with  the  hand,  the  lower 
part  of  the  arm  becomes  a  lever  of  3ie  third  kind ;  the  elbow  is 
the  fulcrum,  the  muscles  of  the  fleshy  part  of  the  arm,  the  power; 
and  as  these  are  nearer  to  the  elbow  than  to  the  hand,  it  is  ne- 
cessary that  their  power  should  exceed  the  weight  to  be  raised. 

Emily.  Is  it  not  surprising  that  nature  should  have  furnished 
us  with  such  disadvantageous  levers  ? 

Mrs,  B.  The  disadvantage,  in  respect  to  power,  is  more  than 
counterbalanced  by  the  convenience  resulting  from  this  structure 
of  the  arm ;  and  it  is  that  no  doubt  which  is  best  adapted  to 
enable  it  to  perform  its  various  functions. 

There  is  one  rule  which  applias  to  every  lever,  which  is  this : 
In  order  to  produce  an  e<iuilibrium,  the  power  must  bear  the 
same  proportion  to  the  weight,  as  the  length  of  the  shorter  arm 
does  to  that  of  the  longer;  as  was  shown  by  Emily  with  the 
weights  of  1/6.  and  of  Sib,    Fig.  3.  plate  4. 

We  have  dwelt  so  long  on  the  lever,  that  we  must  reserve  the 
examination  of  the  other  mechanical  powers,  to  our  next  interview. 

29.  What  remarks  are  made  on  its  employment  in  the  limbs  of  animals  ? 
30.  What  are  the  conditions  of  equilibrium  in  every  lever  ? 


CONVERSATION  V. 

CONTINUED. 


ON  THE  MECHANICAL  POWERS. 

«F  THE  PULLEY. — OF  THE  WHEEL  AND  AXLE.— OF  THE  INCLINED  PLANE. 
— OF  THE  WEDGE.— OP  THE  SCREW. 

MRS.  B. 

The  pulley  is  the  second  mechanical  power  we  are  to  exam- 
ine.    You  both,  I  suppose,  have  seen  a  pulley? 

Caroline.  Yes,  frequently:  it  is  a  circular,  and  flat  piece  of 
wood  or  metal,  with  a  string  which  runs  in  a  groore  round  it :  by 
means  of  which,  a  weight  may  be  pulled  up;  thus  pulleys  are 
used  for  drawing  up  curtains. 

Mrs,  B.  Yes ;  but  in  that  instance  the  pulleys  are  fixed;  that 
is,  they  retain  their  places,  and  merely  turn  round  on  their  axis ; 
these  do  not  increase  the  power  to  raise  the  weights,  as  you  will 
perceive  by  this  figure,  (plate  5.  fig.  1.)  Observe  that  the  fixed 
pulley  is  on  the  same  principle  as  the  lever  of  a  pair  of  scales,  in 
which  the  fulcrum  F  being  m  the  centre  of  gravity,  the  power  P 
and  the  weight  W,  are  equally  distant  from  it,  and  no  advantage 
is  gained. 

JBmily.  Certainly;  if  P  represents  the  power  employed  to 
raise  the  weight  W,  the  power  must  be  greater  than  tne  weight 
in  order  to  move  it.  But  of  what  use  then  is  a  fixed  pulley  in 
mechanics  ? 

Mrs.  B.  Although  it  does  not  increase  the  power,  it  is  fre- 
quently useful  for  altering  its  direction.  A  single  fixed  pulley 
enables  us  to  draw  a  curtain  up,  by  pulling  the  string  connected 
with  it  downwards;  and  we  should  be  at  a  loss  to  accomplish 
this  simple  operation  without  its  assistance. 

Caroline.  There  would  certainly  be  some  difliculty  in  ascend- 
ing to  the  head  of  the  curtain,  in  order  to  draw  it  up.  Indeed  I 
now  recollect  having  seen  workmen  raise  weights  to  a  considera- 
ble height  by  means  of  a  fixed  pulley,  which  saved  them  the 
trouble  of  going  up  themselves. 

31.  Describe  a  pulley,  and  its  use.  32.  What  is  meant  by  a  fixed  pulley, 
and  why  is  not  power  gained  by  its  employment?  (figf.  1.  plate  5.)  33.  Of 
what  use  ia  the  fixed  pulley  ? 


ON   THE    MECHANICAL    POWERS.  6§ 

Mrs,  B,  The  next  figure  represents  a  pulley  which  is  not 
fixed ;  ffig.  2.)  and  thus  situated,  you  will  perceive  that  it  affords 
us  mecnanical  assistance. 

A  is  a  moveable  pulley;  that  is,  one  which  is  attached  to  the 
weight  to  be  raised,  and  which  consequently  moves  up  or  down 
with  it.  There  is  also  a  fixed  pulley  D,  which  is  only  of  use  to 
change  the  direction  of  the  power  P.  Now  it  is  evident  that  the 
velocity  of  the  power,  will  be  double  that  of  the  weight  W ;  for 
if  the  rope  be  pulled  at  P,  until  the  pulley  A  ascends  with  the 
weight  to  the  fixed  pulley  D,  then  botn  parts  of  the  rope,  C  and 
B,  must  pass  over  the  fixed  pulley,  and  consequently  the  hand 
at  P,  will  have  descended  through  a  space  equal  to  those  two  parts; 
but  the  weight  will  have  ascended  only  one  half  of  that  distance. 

Caroline.  That  I  understand  :  if  r  drew  the  string  but  one 
inch,  the  weight  would  be  raised  only  half  an  inch,  because  it 
would  shorten  the  strings  B  and  C  half  an  inch  each,  and  conse- 
quently the  pulley  with  the  weight  attached  to  it,  can  be  raised 
only  half  an  inch. 

Emily.  But  I  do  not  yet  understand  the  advantage  of  movea- 
ble pulieys ;  they  seem  to  me  to  increase  rather  than  diminish 
the  difficulty  of  raising  weights,  since  you  must  draw  the  string 
double  the  length  that  you  raise  the  weight;  whilst  with  a  single 
pulley,  or  without  any  pulley,  the  weight  i$  raised  as  much  as 
the  string  is  shortened. 

Mrs.  S.  The  advantage  of  a  moveable  pulley  consists  in  di- 
viding the  difficulty;  we  must,  it  is  true,  draw  twice  the  length 
of  the  string,  but  then  only  half  the  strength  is  required  that 
would  be  necessary  to  raise  the  weight  without  the  assistance  of 
a  moveable  pulley. 

Emily.  So  that  the  difficulty  is  overcome  in  the  same  manner 
as  it  would  be,  by  dividing  the  weight  into  two  equal  parts,  and 
raising  them  successively. 

Mrs.  B.  Exactly.  You  must  observe,  that  with  a  moveable 
pulley  the  velocity  of  the  power,  is  double  that  of  the  weight;  since 
the  power  P  (fig.  2.)  moves  two  inches  whilst  the  weight  W 
moves  one  inch  ;  therefore  the  power  need  not  be  more  than  half 
the  weight,  to  make  their  momentums  equal. 

Caroline.  Pulleys  act  then  on  the  same  principle  as  the  lever; 
the  deficiency  of  weight  in  the  power,  being  compensated  by  its 
superior  velocity,  so  as  to  make  their  momentums  equal. 

Mrs.  B.  You  will  find,  that  all  gain  of  power  in  mechanics  is 
founded  on  the  same  principle. 

Emily.    But  may  it  not  be  objected  to  pulleys,  that  a  longer 

34.  How  is  the  power  gained  by  a  moveable  pulley,  explained  by  means 
of  fig.  2.  plate  5?  35.  What  proportion  must  the  power  bear  to  the  weight 
in  fig.  2,  that  their  momentums  may  be  equal? 


64 


ON    THE    MECHANICAL    POWERS. 


time  is  required  to  raise  a  weight  by  their  aid,  than  without  it? 
for  what  you  gain  in  power,  you  lose  in  time. 

Mrs.  B.  That,  my  dear,*is  the  fundamental  law  in  mecha- 
nics: it  is  the  case  with  the  lever,  as  well  as  the  pulley;  and  you 
will  find  it  to  be  so  with  all  the  other  mechanical  powers. 

Caroline.  I  do  not  see  any  advantage  in  the  mechanical  pow- 
ers then,  if  what  we  gain  by  them  in  one  way,  is  lost  in  another. 

Mrs.  B.  Since  we  are  not  able  to  increase  our  natural  strength, 
18  not  any  instrument  of  obvious  utility,  by  means  of  which  v.e 
may  reduce  the  resistance  or  weight  of  any  body,  to  the  level  of 
that  strength  ?  This  the  mechanical  powers  enable  us  to  accom- 
plish. It  is  true,  as  you  observe,  that  it  requires  a  sacrifice  of  time 
to  attain  this  end,  but  you  must  be  sensible  how  very  advantage- 
ously it  is  exchanged  for  power,  If  one  man  by  his  natural 
strength  could  raise  one  hundred' pounds  only,  it  would  require 
five  such  men  to  raise  five  hundred  pounds ;  and  if  one  man 
per^rms  this  by  the  help  of  a  suitable  engine,  there  is  then  no 
actual  loss  of  time;  as  he  does  the  work  of  five  men,  altholigh  he 
is  five  times  as  long  in  its  accomplishment. 

You  can  now  nnderstand,  that  the  greater  the  number  of 
moveable  pulleys  connected  by  a  string,  the  more  easily  the 
weight  is  raised;  as  tlie  difficulty  is  divided  amongst  the  number 
of  strings,  or  rather  of  parts  into  which  the  string  is  divided,  by 
the  pulleys.  Two,  or  more  pulleys  thus  connected,  form  what 
is  called  a  tackle,  or  system  of  pulleys,  (fig.  3.)  You  may  have 
seen  them  suspended  from  cranes  to  raise  goods  into  warehouses. 

Emily.  When  there  are  two  moveable  pulleys,  as  in  the  fi- 
gure you  have  shown  to  us,  (fig.  3.)  there  must  also  be  two  fixed 
pulleys,  for  the  purpose  of  changing  tlie  direction  of  the  string, 
and  then  the  weight  is  supported  by  four  strings,  and  of  course, 
each  must  bear  only  one  fourth  part  of  the  weight. 

Mrs.  B,  You  are  perfectly  correct,  and  the  rule  for  estimat- 
ing the  power  gainecl  by  a  system  of  pullies,  is  to  count  the 
number  of  strings  by  which  the  weidit  is  supported ;  or,  which 
amounts  to  the  same  thing,  to  multiply  the  number  of  moveable 
pulleys  by  two. 

In  shipping,  the  advantages  of  both  an  increase  of  power,  and 
a  change  of  direction,  by  means  of  pulleys,  are  of  essential  im- 
portance: for  the  sails  are  raised  up  the  masts  by  the  sailors  on 
deck,  from  the  change  of  direction  which  the  pulley  effects,  and 
the  labour  is  facilitated  by  the  mechanical  power  of  a  combina- 
tion of  pulleys. 

36.  What  is  a  fundamental  law  as  respects  power  and  time  ?  37.  If  to 
gain  power  we  must  lose  time,  what  advantage  do  we  derive  from  the  me- 
chanical powers  ?  38.  What  name  is  given  to  two  or  more  pulleys  connected 
by  one  string  i*  39.  How  do  we  estimate  the  power  gained  by  a  system  of 
pulleys  ? 


ON  THE  MECHANICAL  POWERS.  65 

Emily.  But  the  pulleys  on  ship-board  do  not  appear  to  me  to 
be  united  in  the  manner  you  have  shown  us. 

Mrs,  B,  They  are,  I  believe,  generally  connected  as  describ- 
ed in  figure  4,  both  for  nautical,  and  a  variety  of  other  pur- 
poses ;  but  in  whatever  manner  pulleys  are  connected  by  a  single 
string,  the  mechanical  power  is  the  same. 

The  third  mechanical  power,  is  the  wheel  and  axle.  Let  us 
suppose  (plate  6.  fig.  5)  trie  weig;ht  W,  to  be  a  bucket  of  water 
in  a  well,  which  we  raise  by  winding  round  the  axle  the  rope, 
to  which  it  is  attached ;  if  this  be  done  without  a  wheel  to  turn 
the  axle,  no  mechanical  assistance  is  received.  The  axle  with- 
out a  wheel  is  as  impotent  as  a  single  fixed  pulley,  or  a  lever, 
whose  fulcrum  is  in  the  centre :  but  add  the  wheel  to  the  axle, 
and  you  wiM  immediately  find  the  bucket  is  raised  with  much 
less  difficulty.  The  velocity  of  the  circumference  of  the  wheel 
is  as  much  greater  than  that  of  the  axle,  as  it  is  further  from  the 
centre  of  motion ;  for  the  wheel  describes  a  great  circle  in  the 
same  space  of  time  that  the  axle  describes  a  small  one,  therefore 
the  power  is  increased  in  the  same  proportion  as  the  circumfer- 
ence of  the  wheel  is  greater  than  that  of  the  axle.  If  the  veloci- 
ty of  the  wheel  is  twelve  times  greater  than  that  of  the  axle,  a 
power  twelves  times  less  than  the  weight  of  the  bucket,  would 
balance  it;  and  a  small  increase  would  raise  it. 

Emily.  The  axle  acts  the  part  of  the  shorter  arm  of  the  lever, 
the  wheel  that  of  the  longer  arm. 

Caroline.  In  raising  water,  there  is  commonly,  I  believe,  in- 
stead of  a  wheel  attached  to  the  axle,  only  a  crooked  handle, 
which  answers  the  purpose  of  winding  the  rope  round  the  axle, 
and  thus  raising  the  bucket. 

Mrs.  B.  In  this  manner  (fig.  6;)  now  if  you  observe  the  dot- 
ted circle  which  the  handle  describes  in  winding  up  the  rope, 
you  will  perceive  that  the  branch  of  the  handle  A,  which  is  unit- 
ed to  the  axle,  represents  the  spoke  of  a  wheel,  and  answers  the 
purpose  of  an  entire  wheel ;  the  other  branch  B  affords  no  me- 
chanical aid,  merely  serving  as  a  handle  to  turn  the  wheel. 

Wheels  are  a  very  essential  part  of  most  machines ;  they  are 
employed  in  various  ways;  but,  when  fixed  to  the  axle,  their 
mechanical  power  is  always  the  same:  that  is,  as  the  circumfer- 
ence of  the  wheel  exceeds  that  of  the  axle,  so  much  will  the 
energy  of  the  power  be  increased, 

Caroline.  Then  the  larger  the  wheel,  in  proportion  to  the  axle, 
the  greater  must  be  its  effect  ? 

40.  What  is  represented  by  fig.  5.  plate  6  ?  41.  How  does  the  wheel  ope- 
rate in  increasing  power  ?  42.  How  is  this  compared  with  the  lever  ?  43.  How 
does  a  handle  fixed  to  an  axle,  represent  a  wheel,  fig.  6 .'  44.  How  could 
we  increase  the  power  in  this  instrument  ? 

¥2 


i 


66  ON   THE    MECHANICAL    POWERS. 

Mrs,  B,  Certainly.  If  you  have  ever  seen  any  considerable 
mills  or  manufactures,  you  must  have  admired  the  immense 
wheel,  the  revolution  of  which  puts  the  whole  of  the  machinery 
into  motion;  and  though  so  great  an  effect  is  produced  by  it,  a 
horse  or  two  has  sufficient  power  to  turn  it ;  sometimes  a  stream 
of  water  is  used  for  that  purpose,  but  of  late  years,  a  steam-en- 
gine has  been  found  both  the  most  powerful  and  the  most  conve- 
nient mode  of  turning  the  wheel. 

Caroline.  Do  not  the  vanes  of  a  windmill  represent  a  wheel, 
Mrs.  B..? 

Mrs.  B.  Yes ;  and  in  this  instance  we  have  the  advantage  of 
a  gratuitous  force,  the  wind,  to  turn  the  wheel.  One  of  the 
great  benefits  resulting  from  the  use  of  machinery  is,  that  it 
gives  us  a  sort  of  empire  over  the  powers  of  nature,  and  enables  us 
to  make  them  perform  the  labour  which  would  otherwise  fall  to 
the  lot  of  man.  When  a  current  of  wind,  a  stream  of  water,  or 
the  expansive  force  of  stream,  performs  our  task,  we  have  only 
to  sijperintend  and  regulate  their  operations. 

The  fourth  mechanical  power  is  the  inclined  plane;  this  is 
generally  nothing  more  than  a  plank  placed  in  a  sloping  direc- 
tion, which  is  frequently  used  to  facilitate  the  raising  of  weights, 
to  a  small  height,  such  as  the  rolling  of  hogsheads  or  barrels  into 
a  warehouse.  It  is  not  difficult  to  understand,  that  a  weight 
may  much  more  easily  be  rolled  up  a  slope  than  it  can  be  raised 
the  same  height  perpendicularly.  But  in  this,  as  well  as  the 
other  mechanical  powers,  the  facility  is  purchased  by  a  loss  of 
time  (fig.  7;)  for  tlie  weight,  instead  of  moving  directly  from  A 
to  C,  must  move  from  B  to  C,  and  as  the  length  of  the  plane  is 
to' its  height,  so  much  is  the  resistance  of  the  weight  diminished. 
Emily.  Yes;  for  the  resistance,  instead  of  being  confined  to 
the  short  line  A  C,  is  spread  over  the  long  line  B  C. 

Mrs.  B.  The  wedge,  which  is  the  next  mechanical  power,  is 
usually  viewed  as  composed  of  two  inclined  planes  (fig.  8:)  you 
may  have  seen  wood-cutters  use  it  to  cleave  wood.  The  resist- 
ance consists  in  the  cohesive  attraction  of  the  wood,  or  any  other 
body  which  the  wed^e  is  employed  to  separate ;  the  advantage 
gained  by  this  power  is  differently  estimated  by  philosophers;  but 
one  thing  is  certain,  its  power  is  increased,  in  proportion  to  the 
decrease  of  its  thickness,  compared  with  its  length.  The  wedge 
is  a  very  powerful  instrument,  but  it  is  always  driven  forward 
by  blows  from  a  hammer,  or  some  other  body  having  considera- 
ble momentum. 

Emily,    The  wedge,  then,  is  rather  a  compound  than  a  dis- 

45.  What  other  forces  besides  the  power  of  men,  do  we  employ  to  move 
siachines?  46.  What  will  serve  as  an  example  of  an  inclined  plane?  47.  In 
what  proportion  does  it  gain  power?  (fig.  7.)  48.  To  what  is  the  wedge 
aompared  ?  (fig,  8.)    49.  How  does  its  power  increase  ? 


ON   THE    MECHANICAL    POWERS.  67 

tinct  mechanical  power,  since  it  is  not  propelled  by  simple 
pressure,  or  weight,  like  the  other  powers. 

Mrs.  B.  It  IS  so.  All  cutting  instruments  are  constructed 
upon  the  principle  of  the  inclined  plane,  or  the  wedge :  those 
that  have  but  one  edge  sloped,  like  the  chisel,  may  be  referred 
to  the  inclined  plane ;  whilst  the  axe,  the  hatchet,  and  the  knife, 
(when  used  to  split  asunder)  are  used  as  wedges. 

Caroline.  But  a  knife  cuts  best  when  it  is  drawn  across  the 
substance  it  is  to  divide.  We  use  it  thus  in  cutting  meat,  we 
do  not  chop  it  to  pieces. 

Mrs.  B.  The  reason  of  this  is,  that  the  edge  of  a  knife  is 
really  a  very  fine  saw,  and  therefore  acts  best  when  used  like 
that  instrument. 

The  screw,  which  is  the  last  mechanical  power,  is  more  com- 
plicated than  the  others.  You  will  see  by  this  figure,  (fig.  9. ) 
that  it  is  composed  of  two  parts,  the  screw  and  the  nut.  The 
screw  S  is  a  cylinder,  with  a  spiral  protuberance  coiled  round  it, 
called  the  thread ;  the  nut  N  is  perforated  to  receive  the  screw, 
and  the  inside  of  the  nut  has  a  spiral  groove,  made  to  fit  the  spi- 
ral thread  of  the  screw. 

Caroline.  It  is  just  like  this  little  box,  the  lid  of  which  screws 
«n  the  box  as  you  have  described;  but  what  is  this  handle  L 
which  projects  from  the  nut  ? 

Mrs.  B.  It  is  a  lever,  which  is  attached  to  the  nut,  without 
which  the  screw  is  never  used  as  a  mechanical  power.  The 
power  of  the  screw,  complicated  as  it  appears,  is  referable  to  one 
of  the  most  simple  of  the  mechanical  powers ;  which  of  them  do 
you  think  it  is  ? 

Caroline.  In  appearance,  it  most  resembles  the  wheel  and 
axle. 

Mrs.  B.  The  lever,  it  is  true,  has  the  effect  of  a  wheel,  as  it 
is  the  means  by  which  you  turn  the  nut,  or  sometimes  the  screw, 
round  ;  but  the  lever  is  not  considered  as  composing  a  part  of  the 
screw,  thoudi  it  is  true,  that  it  is  necessarily  attached  to  it. 

Emily.  The  spiral  thread  of  the  screw  resembles,  I  think,  an 
inclined  plane :  it  is  a  sort  of  slope,  by  means  of  which  the  nut 
ascends  more  easily  than  it  would  do  if  raised  perpendicularly; 
and  it  serves  to  support  it  when  at  rest. 

Mrs.  B.  Very  well :  if  you  cut  a  slip  of  paper  in  the  form  of 
an  inclined  plane,  and  wind  it  round  your  pencil,  which  will 

50.  Why  is  it  rather  a  compound  than  a  simple  power?  51.  What  com- 
mon instruments  act  upon  the  principle  of  the  inclined  plane,  or  the  wedge  ? 
52.  Why  does  a  knife  cut  best  when  drawn  across  ?  53.  The  screw  has  tw« 
essential  parts  ;  what  are  theyj*  54.  What  other  instrument  is  used  to  turn 
the  screw.''  55.  How  can  you  compare  the  screw  with  an  iiicliaed  plane? 
Fig.  10. 


68  ON   THE   MECHANICAL   POWERS, 

represent  the  cylinder,  you  will  find  that  it  makes  a  spiral  line,, 
corresponding  to  the  spiral  protuberance  of  the  screw.  (Fig.  10.) 

Emily.  Very  true ;  the  nut  then  ascends  an  inclined  plane, 
but  ascends  it  in  a  spiral,  instead  of  a  straight  line :  the  closer 
the  threads  of  the  screw,  the  more  easy  the  ascent:  it  is  like 
having  shallow,  instead  of  steep  steps  to  ascend. 

Mrs.  B.  Yes ;  excepting  that  the  nut  takes  no  steps,  as  it 
gradually  winds  up  or  down ;  then  observe,  that  the  closer  the 
threads  of  the  screw,  the  less  is  its  ascent  in  turning  round,  and 
the  greater  is  its  power ;  so  that  we  return  to  the  old  principle, — 
what  is  saved  in  power  is  lost  in  time. 

Emily.  Cannot  the  power  of  the  screw  be  increased  also,  by 
lengthening  the  lever  attached  to  the  nut  ? 

Mrs.  B.  Certainly.  The  screw,  with  the  addition  of  the 
lever,  forms  a  very  powerful  machine,  employed  either  for  com- 
pression or  to  raise  heavy  weights.  It  h  used  by  book-binders, 
to  press  the  leaves  of  books  together;  it  is  used  also  in  cider  and 
wine  presses,  in  coining,  and  for  a  variety  of  other  purposes. 

Emily.  Pray  Mrs.  B,  by  what  rule  do  you  estimate  tne  power 
of  the  screw  ? 

Mrs.  B.  By  measuring  the  circumference  of  the  circle,  which 
the  end  of  the  lever  would  form  in  one  whole  revolution,  and 
comparing  this  with  the  distance  from  the  centre  of  one  thread  of 
the  screw,  to  that  of  its  next  contiguous  turn ;  for  whilst  the  lever 
travels  that  whole  distance,  the  screw  rises  or  falls  only  through 
the  distance  from  one  coil  to  another, 

Caroline  I  think  that  I  have  sometimes  seen  the  lever  attach- 
ed to  the  screw,  and  not  to  the  nut,  as  it  is  represented  in  the 
figure. 

Mrs.  B.  This  is  frequently  done,  but  it  does  not  in  any 
degree  affect  the  power  of  the  instrument. 

All  machines  are  composed  of  one  or  more  of  these  six  me- 
chanical powers  we  have  examined ;  I  have  but  one  more  remark 
to  make  to  you  relative  to  them,  which  is,  that  friction  in  a  con- 
siderable degree  diminishes  their  force :  allowance  must  there- 
fore always  be  made  for  it,  in  the  construction  of  machinery. 

Caroline,  By  friction,  do  you  mean  one  part  of  the  machine 
rubbing  against  another  part  contiguous  to  it } 

Mrs.  B.  Yes ;  friction  is  the  resistance  which  bodies  meet 
with  in  rubbing  against  each  other;  there  is  no  such  thing  as 
perfect  smoothness  or  evenness  in  nature ;  polished  metals,  though 
they  wear  that  appearance  more  than  most  other  bodies,  are  far 

56.  By  what  two  means  may  the  power  of  the  screw  be  increased  ?  57.  How 
do  we  estimate  the  power  gained  by  the  screw  ?  58.  Is  the  lever  always  at- 
tached to  the  nut,  as  in  the  figure .?  59.  What  is  said  respecting  the  composi- 
tion of  all  machines,  and  for  what  must  allowance  always  be  made  in  eatimat 
ing  their  power  ^ 


ON   THE    MECHANICAL    POWERS.  69 

from  really  possessing  it ;  and  their  inequalities  may  frequently 
be  perceived  through  a  good  magnifying  glass.  When,  there- 
fore, the  surfaces  of  the  two  bodies  come  in  contact,  the  promi- 
nent parts  of  the  one,  will  often  fall  into  the  hollow  parts  of  the 
other,  and  occasion  more  or  less  resistance  to  motion. 

Caroline.  But  if  a  machine  is  made  of  polished  metal,  as  a 
watch  for  instance,  the  friction  must  be  very  trifling  ? 

Mrs.  B.  In  proportion  as  the  surfaces  of  bodies  are  well 
polished,  the  friction  is  doubtless  diminished;  but  it  is  always 
considerable,  and  it  is  usually  computed  to  destroy  one-third  of 
the  power  of  a  machine.  Oil  or  grease  is  used  to  lessen  friction : 
it  acts  as  a  polish,  by  filling  up  the  cavities  of  tlie  rubbing  sur- 
faces, and  thus  making  them  slide  more  easily  over  each  other. 

Caroline.  Is  it  for  this  reason  that  wheels  are  greased,  and 
the  locks  and  hinges  of  doors  oiled  ? 

Mrs.  B.  Yes  ;  in  these  instances  the  contact  of  the  rubbing 
surfaces  is  so  close,  and  they  are  so  constantly  in  use,  that  they 
require  to  be  frequently  oiled,  or  a  considerable  degree  of  fric- 
tion is  produced. 

There  are  two  kinds  of  friction ;  the  first  is  occasioned  by  the 
rubbing  of  the  surfaces  of  bodies  against  each  other,  the  second, 
by  the  rolling  of  a  circular  body;  as  that  of  a  carriage  wheel  upon 
tne  ground :  the  friction  resulting  from  the  first  is  much  the  most 
considerable,  for  great  force  is  required  to  enable  the  sliding 
body  to  overcome  the  resistance  which  the  asperities  of  the  sur- 
faces in  contact  oppose  to  its  motion,  and  it  must  be  either  lifted 
over,  or  break  through  them ;  whilst,  in  the  second  kind  of  fric- 
tion, the  rough  parts  roll  over  each  other  with  comparative  facility; 
hence  it  is,  that  wheels  are  often  used  for  the  sole  purpose  of 
diminishing  the  resistance  from  friction. 

Emily.  This  is  one  of  the  advantages  of  carriage  wheels,  is 
it  not  ? 

Mrs.  B.  Yes ;  and  the  larger  the  circumference  of  the  wheel 
the  more  readily  it  can  overcome  any  considerable  obstacles, 
such  as  stones,  or  inequalities  in  the  road.  When,  in  descend- 
ing a  steep  hill,  we  fasten  one  of  the  wheels,  we  decrease  the 
velocity  of  the  carriage,  by  increasing  the  friction. 

Caroline.  That  is  to  say,  by  converting  the  rolling  friction 
into  the  rubbing  friction.  And  when  you  had  casters  put  to  the 
legs  of  the  table,  in  order  to  move  it  more  easily,  you  changed 
the  rubbing  into  the  rolling  friction. 

Mrs.  B.  There  is  another  circumstance  which  we  have  alrea- 
dy noticed,  as  diminishing  the  motion  of  bodies,  and  which  great- 

60.  What  is  meant  by  friction,  and  what  causes  it  ?  61.  How  may  friction 
be  diminished?  62.  Friction  is  of  two  kinds,  what  are  they?  63.  For  what 
purpose  are  wheels  often  used  ?  64.  When  is  the  friction  of  a  carriage  wheel 
changed  from  the  rolling  to  the  rubbing  friction  ? 


70   CAUSES  OF  THE  MOTION  OF  THE  HEAVENLY  BODIES. 

ly  affects  the  power  of  machines.  This  is  the  resistance  of  the 
medium,  in  which  a  machine  is  worked.  All  fluids,  whether 
clastic  like  air,  or  nonelastic  like  water  and  other  liquids,  are 
called  mediums;  and  their  resistance  is  proportioned  to  their 
density  j  for  the  more  matter  a  body  contains,  the  greater  the 
resistance  it  will  oppose  to  the  motion  of  another  body  striking 
against  it. 

Emily.  It  would  then  be  much  more  difficult  to  work  a  ma- 
chine under  water  than  in  the  air  ? 

Mrs,  B.  Certainly,  if  a  machine  could  be  worked  in  vacuo, 
and  without  friction,  it  would  not  be  impeded,  but  this  is  unat- 
tainable ;  a  considerable  reduction  of  power  must  therefore  be 
allowed  for,  from  friction  and  the  resistance  of  the  medium. 

We  shall  here  conclude  our  observations  on  the  mechanical 
powers.  At  our  next  meeting  I  shall  endeavour  to  give  you  an 
explanation  of  the  motion  of  the  heavenly  bodies. 


CONVERSATION  VI. 


CAUSES  OF  THE  MOTION  OF  THE  HEAVENLY 
BODIES. 

OF  THE  earth's  ANNUAL   MOTION. — OE   THE  PLANETS   AND  THEIR  MO- 
TION.— OF  THE  DIURNAL  MOTION  OF  THE  EARTH  AND  PLANETS. 

CAROLINE. 

I  AM  come  to  you  to-day  quite  elated  with  the  spirit  of  oppo- 
sition, Mrs.  B. ;  for  I  have  discovered  such  a  powerful  objection 
to  your  theory  of  attraction,  that  I  doubt  whether  even  vour  con- 
juror Newton,  with  his  magic  wand  of  gravitation,  will  be  able 
to  dispel  it. 

Mrs.  B.    Well,  my  dear,  pray  what  is  this  weighty  objection  ? 

Caroline,  You  say  that  the  earth  revolves  in  its  orbjt  round 
the  sun  once  in  a  year,  and  that  bodies  attract  in  proportion  to 

65.  What  is  a  medium,  and  in  what  proportion  does  it  diminish  motion  ? 
66.  Under  what  circumstances  must  a  body  be  placed,  in  order  to  move  with- 
eut  impediment  ? 


^      CAUSES  or  THE  MOTION  OF  THE  HEAVENLY  BODIES.    71 

the  quantity  of  matter  they  contain;  now  we  all  know  the  sun  to 
be  much  larger  than  the  earth :  why,  therefore,  does  it  not  draw 
the  earth  into  itself;  you  will  not,  I  suppose,  pretend  to  say  that 
we  are  falling  towards  the  sun  ? 

Emily.  However  plausible  your  objection  appears,  Caroline, 
I  think  you  place  too  much  reliance  upon  it :  when  any  one  has 
given  such  convincing  proofs  of  sagacity  and  wisdom  as  Sir  Isaac 
Newton,  when  we  find  that  his  opinions  are  universally  received 
and  adopted,  is  it  to  be  expected  that  any  objection  we  can  ad- 
vance should  overturn  them  ? 

Caroline.  Yet  I  confess  that  I  am  not  inclined  to  yield  impli- 
cit faith  even  to  opinions  of  the  great  Newton :  for  what  pur- 
pose are  we  endowed  with  reason,  if  we  are  denied  the  privilege 
of  makin»  use  of  it,  by  judging  for  ourselves. 

Mrs.  S.  It  is  reason  itself  which  teaches  us,  that  when  we, 
novices  in  science,  start  objections  to  theories  established  by  men 
of  knowledge  and  wisdom,  we  should  be  diffident  rather  of  our 
own  than  of  their  opinion.  I  am  far  from  wishing  to  lay  the 
least  restraint  on  your  questions ;  you  cannot  be  better  convinced 
of  the  truth  of  a  system,  than  by  finding  that  it  resists  all  your 
attacks,  but  I  would  advise  you  not  to  advance  your  objections 
with  so  much  confidence,  in  order  that  the  discovery  of  their 
fallacy  may  be  attended  with  less  mortification.  In  answer  to 
that  you  have  just  proposed,  I  can  only  say,  that  the  earth  really 
is  attracted  by  the  sun. 

Caroline.  Take  care,  at  least,  that  we  are  not  consumed  by 
him,  Mrs.  B. 

Mrs,  B.  We  are  in  no  danger ;  but  Newton,  our  magician, 
as  you  are  pleased  to  call  him,  cannot  extricate  himself  from 
this  difficulty  without  the  aid  of  some  cabalistical  figures,  which 
I  must  draw  for  him. 

Let  us  suppose  the  earth,  at  its  creation,  to  have  been  pro- 
jected forwards  into  universal  space :  we  know  that  if  no  obsta- 
cle impeded  its  course  it  would  proceed  in  the  same  direction, 
and  with  a  uniform  velocity  for  ever.  In  fig.  1.  plate  6,  A  re- 
presents the  earth,  and  S  the  sun.  We  shall  suppose  the  earth 
to  be  arrived  at  the  point  in  which  it  is  represented  in  the  figure, 
having  a  velocity  which  would  carry  it  on  to  B  in  the  space  of 
one  month;  whilst  the  sun's  attraction  would  bring  it  to  C  in  the 
same  space  of  time.  Observe  that  the  two  forces  of  projection 
and  attraction  do  not  act  in  opposition,  but  perpendicularly,  or 
at  a  light  angle  to  each  other.  Can  you  tell  me  now,  how  the 
earth  will  move  ? 

Emily.    I  recollect  your  teaching  us  that  a  body  acted  upom 

1.  What  revolution  does  the  earth  perform  in  a  year?  2.  Had  the  earth 
received  a  projectile  force  only,  at  the  time  of  ita  creation,  how  would  it  hare 
mored  ? 


72     CAUSES  or  the  motion  of  the  heavenly  bodies. 

by  two  forces  perpendicular  to  each  other,  would  move  in  the 
diagonal  of  a  parallelogram;  if,  therefore,  I  complete  the  paral-l 
lelogram,  by  drawing  the  lines  C  D,  B  D,  the  earth  will  move  in  1 
the  diagonal  AD. 

Mrs,  B,  A  ball  struck  by  two  forces  acting  perpendicularly ' 
to  each  other,  it  is  true,  moves  in  the  diagonal  of  a  parallelogram;  i 
but  you  must  observe  that  the  force  of  attraction  is  continually ; 
acting  upon  our  terrestrial  ball,  and  producing  an  incessant  de- 1 
viation  from  its  course  in  a  right  line,  which  converts  it  into  that  \ 
of  a  curve-line ;  every  point  of  which  may  be  considered  as  con- 1 
stituting  the  diagonal  of  an  infinitely  small  parallelogram.  i 

Let  us  detain  the  earth  a  moment  at  the  point  D,  and  consider 
how  it  will  be  affected  by  the  combined  action  of  the  two  forces 
in  its  new  situation.  It  still  retains  its  tendency  to  fly  off  in  a 
straight  line ;  but  a  straight  line  would  now  carry  it  away  to  F, 
whilst  the  sun  would  attract  it  in  the  direction  D  S ;  how  then 
will  it  proceed  ? 

Emily.  It  will  go  on  in  a  curve-line,  in  a  direction  between 
tliat  of  the  two  forces. 

Mrs.  JS.  In  order  to  know  exactly  what  course  the  earth  will 
foUow,  draw  another  parallelogram  similar  to  the  first,  in  which 
the  line  D  F  describes  the  force  of  projection,  and  the  line  D  S 
that  of  attraction ;  and  you  will  find  that  the  earth  will  proceed] 
in  the  curve-line  D  G.  I 

Caroline.  You  must  now  allow  me  to  draw  a  parallelogram, 
Mrs.  B.  Let  me  consider  in  what  direction  will  the  force  of 
projection  now  impel  the  earth. 

Mrs.  B,  First  draw  a  line  from  the  earth  to  the  sun  repre- 
senting the  force  of  attraction ;  then  describe  the  force  of  pro- 
jection at  a  right  angle  to  it. 

Caroline,  The  earth  will  then  move  in  the  curve  G  I,  of  the 
parallelogram  G  H  I  K. 

Mrs.  S.  You  recollect  that  a  body  acted  upon  by  two  forces, 
moves  through  a  diagonal,  in  the  same  time  tnat  it  would  have 
moved  through  one  of  the  sides  of  the  parallelogram,  were  it 
acted  upon  by  one  force  only.  The  earth  nas  passed  through  the 
diagonals  of  these  three  parallelograms,  in  the  space  of  three 
months,  and  has  performed  one  quarter  of  a  circle;  and  on  the 
same  principle  it  will  go  on  till  it  has  completed  the  whole  of 
the  circle.  It  will  then  recommence  a  course,  which  it  has  pur- 
sued ever  since  it  first  issued  from  the  hand  of  its  Creator,  and 

3.  What  do  the  lines  A  B,  and  A  C,  represent  in  fig.  1.  plate  6?  4.  What 
kave  you  been  taught  respecting  a  body  acted  upon  by  two  forces  at  right 
angles  with  each  other  ?  5.  How  does  tiie  force  of  gravity  change  the  diago- 
nal into  a  curved  line  ?  6.  Describe  the  operation  of  the  forces  of  prqiection 
and  of  gravity  as  illustrated  by  the  parallelograms  in  the  figure  ?  7.  What  is 
the  law  respecting  the  time  required  for  motion  in  th«  diagonal  ? 


Plate  VI. 


A       Fi^.l. 


CAUSES    OF   THE    MOTION    OF    THE    HEAVENLY    BODIES.        / 1? 

w  hich  there  is  every  reason  to  suppose  it  will  continue  to  follow, 
as  long  as  it  remains  in  existence. 

Emily.  Wliat  a  grand  and  beautiful  effect  resulting  from 
so  simple  a  cause  ! 

Caroline.  It  affords  an  example,  on  a  magnificent  scale,  of  the 
.  urvilinear  motion,  which  you  taught  us  in  mechanics.  The  attrac* 
tion  of  the  sun  is  the  centripetal  force,  which  confines  the  earth 
to  a  centre;  and  the  impulse  of  projection,  the  centrifugal  force, 
which  iinpels  the  earth  to  quit  the  sun,  and  fly  off  in  a  tangent. 

Mrs.  JD.  Exactly  so.  A  simple  mode  of  illustrating  the  ef- 
fect of  these  combined  forces  on  the  earth,  is  to  cut  a  slip  of  card 
in  the  form  of  a  carpenter's  square,  as  A,  B,  C ;  (fig.  2.  plate  6.) 
the  point  B  will  be  a  right  angle,  the  sides  of  the  square  being 
perpendicular  to  each  other;  after  having  done  this  you  are  to  de- 
scribe a  small  circle  at  the  angular  point  B,  representing  the 
earth,  and  to  fasten  the  extremity  of  one  of  the  leffs  of  the  souare 
to  a  fixed  point  A,  which  we  shall  consider  as  tlie  sun.  Thus 
situated,  the  two  sides  of  the  square  will  represent  both  the  cen- 
trifugal and  centripetal  forces  ;  A  B,  representing  the  centripetal, 
and  B  C,  tlie  centrifugal  force ;  if  you  now  draw  it  round  the 
fixed  point,  you  will  see  how  the  direction  of  the  centrifugal 
force  varies,  constantly  forming  a  tangent  to  the  circle  in  which 
the  earth  moves,  as  it  is  constantly  at  a  right  angle  with  the 
centripetal  force. 

Emily.  The  earth  then,  gravitates  towards  the  sun,  without 
the  slightest  danger  either  of  approaching  nearer,  or  receding 
further  from  it.  How  admirably  this  is  contrived  !  If  the  two 
forces  which  produce  this  curved  motion,  had  not  been  so  accu- 
rately adjusted,  one,  would  ultimately  have  prevailed  over  the 
other,  and  we  should  either  have  approached  so  near  the  sun  as 
to  have  been  burnt,  or  have  receded  so  far  from  it  as  to  have 
been  frozen. 

Mrs.  B.  What  will  you  say,  my  dear,  when  I  tell  you,  that 
these  two  forces  are  not,  in  fact,  so  proportioned  as  to  produce 
circular  motion  in  the  earth  ?  We  actually  revolve  round  the 
sun  in  an  eliptical  or  oval  orbit,  the  Sun  being  situated  in  one  of 
the  foci  or  centres  of  the  oval,  so  that  the  sun  is  at  some  periods 
much  nearer  to  the  earth,  than  at  others. 

Caroline.  You  must  explain  to  us,  at  least,  in  what  manner 
we  avoid  the  threatened  destruction. 

8.  What  portion  of  a  year  is  represented  by  the  three  diagonals  in  the  figure  f 
9.  How  will  what  you  have  learned  respecting  motion  in  a  curve,  apply  to 
the  earth's  motion  ?  10.  In  what  form  are  you  directed  to  cut  a  piece  of 
card  to  aid  in  illustrating  the  two  forces  acting  upon  the  earth?  11.  How 
must  you  apply  it  to  this  purpose?  (fig.  2.  plate  6.)  12.  If  these  two  forces 
iid  not  exactly  balance  each  other,  what  would  result  ?     13.  Does  the  earth 

volve  in  a  circular  orbit  ?     14.  What  results  from  its  motion  in  an  eclipse  f 

G 


74        CAUSES    OF   THE    MOTION    OF   THE    HEAVENLY   BODIES. 

Mrs,  B,  Let  us  suppose  that  when  the  earth  is  at  A,  (fig.  S.) 
its  projectile  force  should  not  have  given  it  a  velocity  sufficient 
to  counterbalance  that  of  gravity,  so  as  to  enable  these  powers 
conjointly  to  carry  it  round  the  sun  in  a  circle ;  the  earth,  instead 
of  describing  the  line  A  C,  as  in  the  former  figure,  will  approach 
nearer  the  sun  in  the  line  A  B. 

Caroline.  Under  these  circumstances,  I  see  not  what  is  to 
prevent  our  approaching  nearer  and  nearer  the  sun,  till  we  fall 
into  it :  for  its  attraction  increases  as  we  advance  towards  it,  and 
produces  an  accelerated  velocity  in  the  earth,  which  increases 
the  danger. 

Mrs.  B,  There  is  another  seeming  danger,  of  which  you  are 
not  aware.  Observe,  that  as  the  earth  approaches  the  sun,  the 
direction  of  its  projectile  force  is  no  longer  perpendicular  to  that 
of  its  attraction,  but  inclines,  more  nearly  to  it.  When  the  earth 
reaches  that  part  of  its  orbit  at  B,  the  force  of  projection  would 
carry  it  to  D,  which  brings  it  nearer  the  sun  instead  of  bearing 
it  away  from  it. 

Emily.  If,  then,  we  are  driven  by  one  power,  and  drawn  by 
the  other  to  this  centre  of  destruction,  how  is  it  possible  for  us 
to  escape  ? 

Mrs.  B.  A  little  patience,  and  you  will  find  that  we  are  not 
without  resource.  The  earth  continues  approaching  the  sun  with 
a  uniformly  increasing  accelerated  motion,  till  it  reaches  the 
point  E;  in  what  direction  will  the  projectile  force  now  impel  it? 

Emily.  In  the  direction  E  F.  Here  then  the  two  forces  act 
perpendicularly  to  each  other,  the  lines  representing  them  forming 
a  ri^ht  angle,  and  the  earth  is  situated  just  as  it  was  in  the  pre- 
ceding figure;  therefore,  from  this  point,  it  should  revolve  round 
the  sun  in  a  circle. 

Mrs.  B.  No,  all  the  circumstances  do  not  agree.  In  motion 
round  a  centre,  you  recollect  that  the  centrifugal  force  increases 
with  the  velocity  of  the  body,  or  in  other  words,  the  quicker  it 
moves  the  stronger  is  its  tendency  to  fly  off  in  a  right  line. 
When  the  earth,  therefore,  arrives  at  E,  its  accelerated  motion 
will  have  so  far  increased  its  velocity,  and  consequently  its  cen- 
trifugal force,  that  the  latter  will  prevail  over  the  force  of  attrac- 
tion, and  force  the  earth  away  from  the  sun  till  it  reaches  G. 

Caroline.  It  is  thus  then  that  we  escape  from  the  dangerous 
vicinity  of  the  sun ;  and  in  proportion  as  we  recede  from  it,  the 
force  of  its  attraction,  and,  consequently,  the  velocity  of  the 
earth's  motion,  are  diminished. 

13.  What  is  represented  by  the  lines  A  C,  A  B,  in  fig.  3.  plate  6  ?  16.  Were 
the  projectile  force  to  carry  the  earth  from  B  to  D,  (fig.  3.)  what  would  re- 
sult? 17.  When  it  has  arrived  at  E,  what  angle  will  be  formed  by  the  lines 
representing  tlie  two  forces?  18.  What  effect  will  the  accelerated  motion 
the»  produc€  ? 


CAUSES    OF  THE    MOTION    OF  THE    HEAVENLY   BODIES.        75 

Mrs,  B,  Yes.  From  G  the  direction  of  projection  is  towards 
H,  that  of  attraction  towards  S,  and  the  earth  proceeds  between 
them  with  a  uniformly  retarded  motion,  till  it  has  completed  its 
revolution.  Thus  you  see  that  tlie  earth  travels  round  the  sun, 
not  in  a  circle,  but  an  elipsis,  of  which  the  sun  occupies  one  of 
"the  foci;  and  that  in  its  course,  the  earth  alternately  approaches 
and  recedes  from  it,  without  any  danger  of  being  either  swallow- 
ed up,  or  being  entirely  carried  away  from  it. 

Caroline.  And  I  observe,  that  what  I  apprehended  to  be  a 
dangerous  irregularity,  is  the  means  by  whicn  the  most  perfect 
order  and  harmony  are  produced. 

Emily.  The  earth  travels  then  at  a  very  unequal  rate,  its 
velocity  being  accelerated  as  it  approaches  the  sun,  and  retarded 
as  it  recedes  from  it. 

Mrs.  B.  It  is  mathematically  demonstrable,  that,  in  moving 
round  a  point  towards  which  it  is  attracted,  a  body  passes  ovfer 
equal  areas,  in  equal  times.  The  whole  of  the  space  contained 
within  the  earth's  orbit,  is  in  fig.  4,  divided  into  a  number  of 
areas  or  surfaces ;  1,  2,  3,  4,  &c.  all  of  which  are  of  equal  dimen- 
sions, though  of  very  different  forms  ;  some  of  them,  you  see,  are 
long  and  narrow,  others  broad  and  short :  but  they  each  of  them 
contain  an  equal  quantity  of  space.  An  imaginary  line  drawn 
from  the  centre  of  the  earth  to  that  of  the  sun,  and  keeping  pace 
with  the  earth  in  its  revolution,  passes  over  equal  areas  in  equal 
times ;  that  is  to  say,  if  it  is  a  month  going  from  A  to  B,  it  will 
be  a  month  going  from  B  to  C,  and  another  from  C  to  E,  and  so 
on;  and  the  areas  A  B  S,  B  C  S,  C  E  S,  will  be  equal  to  each 
other,  althougii  the  lines  A  B,  B  C,  C  E,  are  unequal. 

Caroline.  What  long  journeys  the  eailh  has  to  perform  in  the 
course  of  a  month,  in  one  part  of  her  orbit,  and  how  short  they 
are  in  the  other  part ! 

Mrs.  B.  The  inequality  is  not  so  considerable  as  appears  in 
this  figure ;  fnr  the  earth's  orbit  is  not  so  eccentric  as  it  is  there 
described ;  and  in  reality,  differs  but  little  from  a  circle :  that 
part  of  the  earth's  orbit  nearest  the  sun  is  called  its  perihelion, 
that  part  most  distant  from  the  sun,  its  aphelion;  and  the  earth 
is  above  three  millions  of  miles  nearer  the  sun  at  its  perihelion 
than  at  its  aphelion. 

Emily.  I  think  I  can  trace  a  consequence  from  these  differ- 
ent situations  of  the  earth ;  are  not  they  the  cause  of  summer 
and  winter  ? 

19.  What  is  the  form  of  the  earth's  orbit,  and  what  circumstances  produce 
this  form  ?  20.  What  is  the  consequence  as  regards  the  regularity  ol  the 
earth's  motion  ?  21.  What  law  governs  as  regards  the  spaces  passed  over,  and 
how  is  this  explained  by  fig.  4.  plate  3  ?  22.  What  is  meant  hy  perihelion^  and 
by  aphelion  ?  23.  What  is  the  difference  of  the  distance  of  the  earth  from  the 
sun,  in  these  two  points  ? 


76        CAUSES  OF  THE  MOTION  OF  THE  HEAVENLY  BODIES. 

Mrs,  B.  On  the  contrary,  during  the  height  of  summer,  the 
earth  is  in  that  part  of  its  orbit  which  is  most  distant  from  the 
sun,  and  it  is  during  the  severity  of  winter,  that  it  approaches 
nearest  to  it. 

Emily.  That  is  very  extraordinary;  and  how  then  do  you 
account  for  the  heat  being  greatest,  when  we  are  most  distant 
from  the  sun  ? 

Mrs.  B.  The  difference  of  the  earth's  distance  from  the  sun 
in  summer  and  winter,  when  compared  with  its  total  distance 
from  the  sun,  is  but  inconsiderable.  The  earth,  it  is  true,  is 
above  three  millions  of  miles  nearer  the  sun  in  winter  than  in 
summer;  but  that  distance,  however  great  it  at  first  appears, 
riinks  into  insignificance  in  comparison  with<95  millions  of  mile^,"*^ 
which  is  our  mean  distance  from  the  sun.  The  change  of  tem- 
perature, arising  from  this  difference,  would  scarcely  be  sensible, 
'^ven  were  it  not  completely  overpowered  by  other  causes  which 
produce  the  variations  of  the  seasons;  but  these  I  shall  defer 
explaining,  till  we  have  made  some  further  observations  on  the 
heavenly  bodies. 

Caroline,  And  should  not  the  sun  appear  smaller  in  summer, 
vvhen  it  is  so  much  further  from  us  ? 

Mrs.  B.  It  actually  does,  when  accurately  measured;  but 
liie  appai^ent  difference  in  size,  is,  I  believe,  not  perceptible  to 
the  naked  eye. 

Emily.  Then,  since  the  earth  moves  with  the  gi-eatest  velo- 
w  ity  in  that  part  of  its  orbit  in  which  it  is  nearest  the  sun,  it 
must  have  completed  its  journey  through  that  half  of  its  orbit,  in 
n  shorter  time  than  through  the  other  ? 

Mrs.  B.  Yes,  it  is  about  seven  days  longer  performing  the 
■iummer-half  of  its  orbit,  than  the  winter-half;  and  the  summers 
;ire  consequently  seven  days  longer  in  the  northern,  than  they 
are  in  the  southern  hemisphere. 

The  revolution  of  all  the  planets  round  the  sun,  is  the  result 
of  the  same  causes,  and  is  performed  in  the  same  manner,  as  that 
of  the  earth. 

Caroline.    Pray  what  are  the  planets  ? 

'Mrs.  B.  Thev  are  those  celestial  bodies,  which  revolve  like 
our  earth,  about  the  sun"^  they  are  supposed  to  resemble  the  earth 
also  in  many  other  respects ;  and  we  are  led  by  analogy,  to  sup- 
pose them  to  be  inhabited  worlds. 

24.  At  what  season  of  the  year  is  it  nearest  to,  and  at  what  furthest  from 
the  sun  ?  25.  What  is  the  mean  distance  of  the  earth  from  the  sun  ?  26.  Why 
is  but  little  effect  produced,  as  regards  temperature,  by  the  change  of  distance  ? 
27.  Has  it  any  influence  on  the  sun's  apparent  size  ?  28.  Are  the  summer  and 
winter,  half  years,  of  the  same  length ;  what  is  their  difierence,  and  what  is 
the  cause  ?     29.  What  are  the  planets  ? 


CAUSES    OF   THE   MOTION    OF   THE    HEAVENLY    BODIES.        /  7 

Caroline.    I  have  heard  so,  but  do  you  not  think  such  an 
opinion  too  great  a  stretch  of  the  imagination  ? 

Mrs»  B.  Some  of  the  planets  are  proved  to  be  larger  than 
the  earth ;  it  is  only  their  immense  distance  from  us,  which  ren- 
ders their  apparent  dimensions  so  small.  /  Now,  if  we  consider 
them  as  enormous  globes,  instead  of  small  twinkling  spots,  we 
shall  be  led  to  suppose  that  the  Almighty  would  not  have  cre- 
ated them  merely  for  the  purpose  of  giving  us  a  little  light  in  the 
night,  as  it  was  formerly  imadned ;  and  we  should  find  it  more 
consistent  with  our  ideas  of  the  Divine  wisdom  and  beneficence, 
to  suppose  that  these  celestial  bodies  should  be  created  for  the 
habitation  of  beings,  who  are,  like  us,  blessed  by  his  providence. 
Both  in  a  moral,  as  well  as  a  physical  point  of  view,  it  appears  to 
me  more  rational  to  consider  the  planets  as  worlds  revolving 
round  the  sun ;  and  the  fixe4  stars  as  other  suns,  each  of  them 
attended  by  their  respective  system  of  planets,  to  wliich  they 
impart  their  influence.  We  have  brought  our  telescopes  to  such 
a  degree  of  perfection,  that  from  the  apnearances  which  the  moon 
exhibits  when  seen  through  them,  we  nave  very  ^ood  reason  to 
conclude  that  it  is  a  habitable  globe :  for  though  it  is  true  that  we 
cannot  discern  its  towns  and  people,  we  can  plainly  perceive  its 
mountains  and  valleys :  and  some  astronomers  have  gone  so  far 
as  to  imagine  that  they  discovered  volcanos. 

Emily.  If  the  fixed  stars  are  suns,  with  planets  revolving 
round  them,  why  should  we  not  see  those  planets  as  well  as  their 
suns? 

Mrs.  B.  In  the  first  place,  we  conclude  that  the  planets  of 
other  systems  (like  those  of  our  own)  are  much  smaller  than  the 
suns  which  give  them  li^ht;  therefore  at  a  distance  so  great  as  to 
make  the  suns  appear  like  fixed  stars,  the  planets  would  be  quite 
invisible.  Secondly,  the  light  of  the  planets  being  only  reflected 
light,  is  much  more  feeble  than  that  of  the  fixed  stars.  There  is 
exactly  the  same  difference  as  between  the  light  of  the  sun  and 
that  of  the  moon;  the  first  being  a  fixed  star,  the  second  a  planet. 

Emily.  But  the  planets  appear  to  us  as  bright  as  the  fixed 
stars,  and  these  you  tell  us  are  suns  like  our  own ;  why  then  do 
we  not  see  them  by  day -light,  when  they  must  be  just  as  lumi- 
nous as  they  are  in  the  night  ? 

Mrs.  B.  Both  are  invisible  from  the  same  cause:  their  light 
is  so  faint,  compared  to  that  of  the  sun,  that  it  is  entirely  effaced 
by  it :  the  light  emitted  by  the  fixed  stars  may  probably  be  as 
great  as  that  of  our  sun,  at  an  equal  distance ;  but  they  being  so 

30.  What  circumstances  render  it  probable  that  they  are  habitable  globes  ? 
31.  What  is  believed  respecting  the  fixed  stars  ?  32.  What  discoveries  have 
been  made  in  the  moon  ?  33.  What  prevents  our  seeing  the  planets,  if  there 
are  any,  which  revolve  round  the  fixed  stars  ?  34.  What  prevents  our  seeinoj 
tlie  stars  and  planets  in  the  day-time? 
G  2 


78   CAUSES  OF  THE  MOTION  OF  THE  HEAVENLY  BODIES. 

much  more  remote,  it  is  diiFused  over  a  greater  space,  and  is  in 
consequence  proportionally  lessened. 

Caroline.  True ;  I  can  see  much  better  by  the  light  of  a  can- 
dle that  is  near  me,  than  by  that  of  one  at  a  great  distance. 
But  I  do  not  understand  what  makes  the  planets  shine  ? 

Mrs.  B.  What  is  that  which  makes  the  gilt  buttons  on  your 
brother's  coat  shine  ? 

Caroline.  The  sun.  But  if  it  was  the  sun  which  made  the 
planets  shine,  we  should  see  them  in  the  day-time,  when  the  sun 
shone  upon  them ;  or  if  the  faintness  of  their  light  prevented  our 
seeing  tliem  in  the  day,  we  should  not  see  them  at  all,  for  the 
sun  cannot  shine  upon  them  in  the  night. 

Mrs.  B.  There  you  are  in  error.  But  in  order  to  explain 
this  to  you,  I  must  first  make  you  acquainted  with  the  various 
motions  of  the  planets. 

You  know,  that  according  to  the  laws  of  attraction,  the  planets 
belonging  to  our  system  all  gravitate  towards  the  sun ;  and  that 
this  force,  combined  with  that  of  projection,  will  occasion  their 
revolution  round  the  sun,  in  orbits  more  or  less  elliptical,  accord- 
ing to  the  proportion  which  these  two  forces  bear  to  each  other. 

But  the  planets  have  also  another  motion :  they  revolve  upon 
their  axis.  The  axis  of  a  planet  is  an  imaginary  line  which 
passes  throueh  its  centre,  and  on  which  it  turns ;  and  it  is  this 
motion  which  produces  day  and  night.  It  is  day  on  that  side  of 
the  planet  which  faces  the  sun ;  and  on  the  opposite  side,  which 
remains  in  darkness,  it  is  night.  Our  earth,  which  M^e  consider 
as  a  planet,  is  24  hours  in  performing  one  revolution  on  its  axis ; 
in  that  period  of  time,  therefore,  we  have  a  day  and  a  ni^ht ; 
hence  this  revolution  is  called  the  earth's  diurnal  or  daily  motion; 
and  it  is  this  revolution  of  the  earth  from  west  to  east  which  pro- 
duces an  apparent  motion  of  the  sun,  moon  and  stars,  in  a  con- 
trary direction.  1 

Let  us  now  suppose  ourselves  to  be  beings  independent  of  any 
planet,  travelling  in  the  skies,  and  looking  upon  the  earth  from  a 
point  as  distant  from  it  as  from  other  planets. 

Caroline.  It  would  not  be  flattering  to  us,  its  inhabitants,  to 
3ee  it  make  so  insignificant  an  appearance. 

Mrs.  B.  To  those  accustomed  to  contemplate  it  in  this  light, 
it  could  never  appear  more  glorious.  We  are  taught  by  science 
to  distrust  appearances;  and  instead  of  considering  the  fixed 
stars  and  planets  as  little  points,  we  look  upon  them  either  as 
brilliant  suns,  or  habitable  worlds;  and  we  consider  the  whole 

35.  What  other  motions  have  the  earth  and  planets,  besides  that  in  their 
orbits  ?  36.  What  is  the  imaginary  line  called,  round  which  they  revolve  ? 
37.  How  does  this  occasion  night  and  day  ?  38.  In  what  direction  does  tlie 
earth  turn  upon  its  axis,  and  what  apparent  motion  of  the  s«n,  moon,  and  staivs 
>  thereby  produceij  ? 


CAUSES    OF   THE    MOTION    OF   THE    HEAVENLY    BODIES.        79 

together  as  forming  one  vast  and  magnificent  system,  worthy  o 
the  Divine  hand  by  which  it  was  created. 

Endly.  I  can  scarcely  conceive  the  idea  of  this  immensity  of 
creation ;  it  seems  too  sublime  for  our  imagination ; — and  to  think 
t4iat  the  goodness  of  Providence  extends  over  millions  of  worlds 
throughout  a  boundless  universe — Ah!  Mrs.  B.,  it  is  we  only 
wlio  become  trjfling  and  insignificant  beings  in  so  magnificent  a 
creation ! 

Mrs.  B.  This  idea  should  teach  us  humility,  but  without 
producing  despondency.  The  same  Almighty  hand  which  guides 
these  countless  worlds  in  their  undeviating  course,  conducts  with 
equal  perfection,  the  blood  as  it  circulates  through  the  veins  of  a 
fly,  and  opens  the  eye  of  the  insect  to  behold  His  wonders. 
Notwithstanding  this  immense  scale  of  creation,  therefore,  we 
need  not  fear  that  we  shall  be  disregarded  or  forgotten. 

But  to  return  to  our  station  in  the  skies.  We  were,  if  you 
recollect,  viewing  the  earth  at  a  great  distance,  in  appearance  a 
little  star,  one  side  illumined  by  the  sun,  the  other  in  obscurity. 
But  would  you  believe  it,  Caroline,  many  of  the  inhabitants  of 
this  little  star  imagine  that  when  that  part  which  they  inhabit  is 
turned  from  the  sun,  darkness  prevails  throughout  the  universe, 
merely  because  it  is  night  with  them ;  whilst,  in  reality,  the  sun 
never  ceases  to  shine  upon  every  planet.  When,  therefore,  these 
little  ignorant  beings  look  around  them  during  their  flight,  and 
behold  all  the  stars  shining,  they  cannot  imagine  why  the  planets, 
which  are  dark  bodies,  should  shine;  concluding,  that  since  the 
sun  does  not  illumine  themselves,  the  whole  universe  must  be  in 
darkness. 

Caroline.  I  confess  that  I  was  one  of  these  ignorant  people  ; 
but  I  am  now  very  sensible  of  the  absurdity  of  such  an  idea.  To 
the  inhabitants  .of"  the  other  planets,  then,  we  must  appear  as  a 
little  star? 

Mrs.  B.  Yes,  to  those  which  revolve  round  our  sun;  for 
since  those  which  may  belong  to  other  systems,  (and  whose  exist- 
ence is  only  hypothetical)  are  invisible  to  us,  it  is  probable  that 
we  also  are  invisible  to  them. 

Emily.  But  they  may  see  our  sun  as  we  do  theirs,  in  appear- 
ance a  fixed  star  ? 

Mrs.  B.  No  doubt ;  if  the  beings  who  inhabit  those  planets 
are  endowed  with  senses  similar  to  ours.  By  the  same  rule  we 
must  appear  as  a  moon  to  the  inhabitants  of  our  moon ;  but  on  a 
larger  scale,  as  the  surface  of  the  earth  is  about  thirteen  times  as 
large  as  that  of  the  moon. 

39.  What  must  be  the  appearance  of  the  earth  to  an  inhabitant  of  one  of 
the  planets  ?  40.  What  the  appearance  of  the  sun  to  tjie  inhabitants  of  pla- 
nets in  other  systems?  40.  What  the  appearance  of  the  earth  to  an  inhabit- 
ant of  the  moon  ? 


80  OF    THE    PLANETS. 

Emily.  The  moon,  Mrs.  B.,  appears  to  mgve  in  a  different 
direction,  and  in  a  different  manner  from  the  stars  ? 

Mrs.  B.  I  shall  defer  the  explanation  of  the  motion" of  the 
moon  till  our  next  interview,  as  it  would  prolong  our  present 
lesson  too  much. 


CONVERSATION  VII. 


OF  THE  PLANETS. 

OF  THE  SATELLITES  OR  MOONS. — GRAVITY  DIMINISHES  AS  THE  SftUARE 
OF  THE  DISTANCE. — OF  THE  SOLAR  SYSTEM. — OF  COMETS. — CONSTHL- 
LATIONS,  SIGNS  OF  THE  ZODIAC. — OF  COPERNICUS,  NEWTON,  &C. 

MRS.  B. 

The  planets  are  distinguished  into  primary  and  secondary. 
Those  wliich  revolve  immediately  about  the  sun  are  called  pri- 
mary. Many  of  these  are  attended  in  their  course  by  smaller 
planets,  which  revolve  round  them :  these  are  called  secondary 
planets,  satellites,  or  moons.  Such  is  our  moon  which  accom- 
panies the  earth,  and  is  carried  with  it  round  the  sun, 

Emily.  How  then  can  you  reconcile  the  motion  of  the  secon- 
dary planets  to  the  laws  of  gravitation ;  for  the  sun  is  much 
larger  than  any  of  the  primary  planets ;  and  is  not  the  power 
of  gravity  proportional  to  the  quantity  of  matter  ? 

Caroline,  Perhaps  the  sun,  though  much  larger,  may  be  less 
dense  than  the  planets.  Fire  you  know,  is  very  light,  and  it 
may  contain  but  little  matter,  though  of  great  magnitude. 

Mrs.  B.  We  do  not  know  of  what  kind  of  matter  the  sun  is 
made ;  but  we  may  be  certain,  that  since  it  is  the  general  centre 
of  attraction  of  our  system  of  planets,  it  must  be  the  body  which 
contains  the  greatest  quantity  of  matter  in  that  system. 

You  must  recollect,  that  the  force  of  attraction  is  not  only 

1 ,  Into  what  two  classes  are  the  planets  divided,  and  how  are  they  distin- 
^lished  .'*  2.  By  what  reasoning  do  you  prove  that  the  sun  contains  a  greater 
quantity  of  matter  than  aay  otber  body  in  the  system } 


OF    THE    PLANETS.  81 

proportional  to  the  quantity  of  matter,  but  to  the  degree  of  prox- 
imity of  the  attractive  boily:  this  power  is  weakened  by  being 
dift'used,  and  tliminishes  as  the  distance  increases. 

Emily,  Then  if  a  phiiiet  was  to  lose  one-half  of  its  quantity 
of  matter,  it  would  lose  one  half  of  its  attractive  power;  and  the 
same  effect  would  be  produced  by  removing  it  to  twice  its 
former  distance  from  the  sun;  that  I  understand. 

Mrs,  B,  Not  so  perfectly  as  you  imagine.  You  are  correct 
as  respects  the  diminution  in  size,  because  the  attractive  force 
is  in  the  same  proportion  as  the  quantity  of  matter;  but  were  you 
to  remove  a  planet  to  double  its  former  distance,  it  would  retain 
but  one-fourth  part  of  its  gravitating  force ;  for  attraction  de- 
creases not  in  proportion  to  the  simple  increase  of  the  distance, 
but  as  the  squares  of  the  distances  increase. 

Caroline,  I  do  not  exactly  comprehend  what  is  meant  by  the 
squares,  in  this  case,  although  I  know  very  well  what  is  in  ge- 
neral intended  by  a  square. 

Mrs.  B,  By  the  square  of  a  number  we  mean  the  product  of 
a  number,  multiplied  by  itself;  thus  two,  multiplied  by  two,  is 
four,  which  is  therefore  the  square  of  two ;  in  like  manner  the 
square  of  three,  is  nine,  because  three  multiplied  by  three,  gives 
that  product. 

Emily,  Then  if  one  planet  is  three  times  more  distant  from 
the  sun  than  another,  it  will  be  attracted  with  but  one-ninth  part 
of  the  force;  and  if  at  four  times  the  distance,  with  but  one-six- 
teenth, sixteen  being  the  square  of  four  ? 

Mrs,  B,  You  are  correct;  the  rule  is,  that  the  attractive  force 
is  in  the  inverse  proportion  of  the  square  of  the  distance.  And  it 
is  easily  demonstrated  by  the  mathematics,  that  the  same  is  the 
case  with  every  power  that  emanates  from  a  centre ;  as  for  ex- 
ample, the  light  from  the  sun,  or  from  any  other  luminous  body, 
decreases  in  its  intensity  at  the  same  rate. 

Caroline.  Then  the  more  distant  planets,  move  much  slower 
in  their  orbits ;  for  their  projectile  force  must  be  proportioned  to 
that  of  attraction  ?  But  I  do  not  see  how  this  accounts  for  the 
motion  of  the  secondary,  round  the  primary  planets,  in  preference 
to  moving  round  the  sun  ? 

Emily,  Is  it  not  because  the  vicinity  of  the  primary  planets, 
renders  their  attraction  stronger  than  that  of  the  sun  ? 

3.  What  two  circumstances  govern  the  force  with  which  bodies  attract  each 
other  ?  4.  Were  a  planet  removed  to  double  its  former  distance  from  the  sun, 
what  would  be  the  effect  upon  its  attractive  force  ?  5.  Why  would  it  be  re^ 
duced  to  one-fourth  f  6.  What  is  meant  by  the  square  of  a  number,  and 
what  examples  can  you  give  ?  7.  What  then  would  be  the  effect  of  removing 
it  to  three,  or  four  times  its  former  distance  ?  8.  How  is  the  rule  upon  this 
subject  expressed  ?  9.  Does  this  apply  to  any  power  excepting  gravitation? 
10.  How  is  it  that  a  secondary  planet  revolves  round  its  primary,  and  is  not 
drawn  off  by  the  sun  ? 


86  OF   THE   PLANETS, 

Mrs.  B,  Exactly  so.  But  since  the  attraction  between  bo- 
dies is  mutual,  the  primary  planets  are  also  attracted  by  th 
satellites  which  revolve  round  them.  The  moon  attracts  the 
earth,  as  well  as  the  earth  the  moon ;  but  as  the  latter  is  the 
smaller  body,  her  attraction  is  proportionally  less;  therefor** 
neither  the  earth  revolves  round  the  moon,  nor  the  moon  round 
the  earth ;  but  they  both  revolve  round  a  point,  which  is  their 
common  centre  oi  gravity,  and  which  is  as  much  nearer  to  the 
earth  than  to  the  moon,  as  the  gravity  of  the  former  exceeds  that 
of  the  latter. 

Emily.  Yes,  I  recollect  your  saying,  ?that  if  two  bodies  were 
fastened  together  by  a  wire  or  bar,  their  common  centre  of  gra- 
vity would  be  in  the  middle  of  the  bar,  provided  the  bodies  were 
of  equal  weight ;  and  if  they  differed  in  weight,  it  would  be  near- 
er the  larger  body.  If  then,  the  earth  and  moon  had  no  projec- 
tile force  which  prevented  their  mutual  attraction  from  bringing 
them  together,  they  w^ould  meet  at  their  common  centre  of  gr 
vity. 

Caroline,  The  earth  then  has  a  great  variety  of  motion,  it 
revolves  round  the  sun,  round  its  own  axis,  and  round  the  point 
towards  which  the  moon  attracts  it. 

Mrs,  B.  Just  so;  and  this  is  the  case  with  every  planet 
which  is  attended  by  satellites.  The  complicated  effect  of  this 
variety  of  motions,  produces  certain  irregularities,  which,  how- 
ever, it  is  not  necessary  to  notice  at  present,  excepting  to  observe 
that  they  eventually  correct  each  other,  so  that  no  permanent 
derangement  exists. 

The  planets  act  on  the  sun,  in  the  same  manner  as  they  are 
themselves  acted  on  by  their  satellites ;  for  attraction,  you  must 
remember,  is  always  mutual ;  but  the  gravity  of  the  planets  (even 
when  taken  collectively)  is  so  trifling  compared  with  that  of  the 
sun,  that  were  they  all  placed  on  the  same  side  of  that  luminary, 
they  would  not  cause  him  to  move  so  much  as  one-half  of  his 
diameter  towards  them,  and  the  common  centre  of  gravity,  would 
still  remain  within  the  body  of  the  sun.  The  planets  do  not, 
therefore,  revolve  round  the  centre  of  the  sun,  but  round  a  point 
at  a  small  distance  from  its  centre,  about  which  the  sun  also  re- 
volves. 

Emily.    I  thought  the  sun  had  no  motion  ? 

Mrs.  B.  You  were  mistaken  ;  for  besides  that  round  the  com- 
mon centre  of  gravity,  which  I  have  just  mentioned,  which  is 
indeed  very  inconsiderable,  he  revolves  on  his  axis  in  about  25 

11.  What  is  said  respecting  the  revolution  of  the  moon,  and  of  the  earth, 
round  a  common  centre  of  gravity  ?  12.  By  what  law  in  mechanics  is  this 
explained  ?  13.  What  motions  then  has  the  earth,  and  are  these  remarks 
confined  to  it  alone  ?  14.  What  effect  have  the  planets  upon  the  sun,  and 
TThat  is  said  of  the  common  centre  of  gravity  of  the  system 


Ficf.  1. 


Fig.  'J. 


OF    THE    PLANETS.  83 

days ;  this  motion  is  ascertained  by  observing  certain  spots  which 
disappear,  and  reappear  regularly  at  stated  times. 

Caroline.  A  planet  has  frequently  been  pointed  out  to  me  in 
the  heavens;  but  I  could  not  perceive  that  its  motion  differed 
from  that  of  the  fixed  stars,  which  only  appear  to  move. 

Mrs,  B,  The  great  distance  of  the  planets,  renders  their 
apparent  motion  so  slow,  that  the  eye  is  not  sensible  of  their 
progress  in  their  orbits,  unless  we  watch  them  for  some  consi- 
derable length  of  time :  but  if  you  notice  the  nearness  of  a  planet 
to  any  particular  fixed  star,  you  may  in  a  few  nights  perceive 
that  it  has  changed  its  distance  from  it,  whilst  the  stars  them- 
selves ahvays  retain  their  relative  situations.  The  most  accu- 
rate idea  I  can  give  you  of  the  situation  and  motion  of  t)ie  pla- 
nets in  their  orbits,  will  be  by  the  examination  of  this  diagram, 
(plate  7.  fig.  1.)  representing  the  solar  system, in  which  you  will 
find  every  planet,  with  its  orbit  delineated. 

Emily.  But  the  orbits  here  are  all  circular,  and  you  said  that 
they  were  eliptical.  The  planets  appear  too,  to  be  moving  round 
the  centre  of  the  sun ;  whilst  3'ou  told  us  that  they  moved  round 
a  point  at  a  little  distance  from  thence. 

Mrs.  B.  The  orbits  of  the  planets  are  so  nearly  circular,  and 
the  common  centre  of  gravity  of  the  solar  system,  so  near  the 
centre  of  the  sun,  that  these  deviations  are  too  small  to  be  re- 
presented. The  dimensions  of  the  planets,  in  their  proportion 
to  each  other,  you  will  find  delineated  in  fig.  2. 

Mercury  is  the  planet  nearest  the  sun  ;  his  orbit  is  consequent- 
ly contained  within  ours ;  his  vicinity  to  the  sun,  prevents  our 
frequently  seeing  him,  so  that  very  accurate  observations  cannot 
be  made  upon  mercury.  He  performs  his  revolution  round  the 
sun  in  about  87  days,  which  is  consequently  the  length  of  his 
year.  The  time  of  his  rotation  on  his  axis  is  not  known ;  his 
distance  from  the  sun  is  computed  to  be  37  millions  of  miles,  and 
his  diameter  3180  miles.  The  heat  of  this  planet  is  supposed  to 
be  so  great,  that  water  cannot  exist  there  but  in  a  state  of  vapour, 
and  that  even  quicksilver  would  be  made  to  boil. 

Caroline.    Oh,  what  a  dreadful  climate ! 

Mrs.  B.  Though  we  could  not  live  there,  it  may  be  perfectly 
adapted  to  other  beings,  destined  to  inhabit  it;  or  he  who  created 
it  may  have  so  modified  the  heat,  by  provisions  of  which  we  are 
ignorant,  as  to  make  it  habitable  even  by  ourselves. 

Venus,  the  next  in  the  order  of  planets,  is  68  millions  of  miles 
from  the  sun :  she  revolves  about  her  axis  in  23  hours  and  21 

15.  What  other  motion  has  the  sun,  and  how  is  it  proved  ?  16.  How  may 
you  observe  the  motion  of  a  planet,  by  means  of  a  fixed  star  ?  17.  What  is 
represented  by  fig.  1.  plate  7  ?  18.  Why  are  the  orbits  represented  as  circu- 
lar? 19.  In  what  order  do  the  planets  increase  in  size  as  represented,  fig. 
2.  plate  7  ?    20.  Wliat  are  we  told  respecting  Mercury  ? 


84  OF   THE    PLANETS. 

minutes,  and  goes  round  the  sim  in  244  days,  17  hours.  The 
orbit  of  Venus  is  also  within  ours ;  during  nearly  one-half  of  her 
course  in  it,  we  see  her  before  sun-rise,  and  she  is  then  called  the 
morning  star ;  in  the  other  part  of  her  orbit  she  rises  later  than 
the  sun. 

Caroline.  In  that  case  we  cannot  see  her,  for  she  must  rise 
in  the  day  time  ? 

Mrs.  A  True;  but  when  she  rises  later  than  the  sun,  she 
also  sets  later;  so  that  we  perceive  her  approaching  the  horizon 
after  sun-set:  she  is  then  called  Hesperus,  or  the  evening  star. 
Do  you  recollect  those  beautiful  lines  of  Milton 

No-w  came  still  evening  on,  and  twilight  gray 
Had  in  her  sober  livery  all  things  clad ; 
Silence  accompanied ;  for  bea^t  and  bird, 
They  to  their  grassy  couch,  these  to  their  nests 
Were  slunk,  all  but  the  wakeful  nightingale ; 
She  all  night  long  her  amorous  descant  sung ; 
Silence  was  pleas'd ;  now  glowed  the  firmament 
"With  living  sapphires.     Hesperus  that  led 
The  starry  host,  rode  brightest,  till  the  moon 
Rising  in  clouded  majesty,  at  length 
Apparent  queen  unveil'd  her  peerless  light, 
And  o'er  the  dark  her  silver  mantle  threw. 

The  planet  next  to  Venus  is  tlie  Earth,  of  which  we  shall  soon 
speak  at  full  length.  At  present  I  shall  only  observe  that  we  are 
95  millions  of  miles  distant  from  t!ie  sun,  that  we  perform  our 
annual  revolution  in  365  days  5  hours  and  49  minutes;  and  are 
attended  in  our  course  by  a  single  moon. 

Next  follows  Mars.  He  can  never  come  between  us  and  the 
sun,  like  Mercury  and  Venus ;  his  motion  is,  however,  very  per- 
ceptible, as  he  may  be  traced  to  different  situations  in  the  hea- 
vens; his  distance  from  the  sun  is  144  millions  of  miles ;  he  turns 
round  his  axis  in  24  hours  and  39  minutes;  and  he  performs  his 
annual  revolution,  in  about  687  of  our  days:  his  diameter  is 
4120  miles.  Then  follow  four  very  small  planets,  Juno,  Ceres, 
Pallas  and  Vesta,  which  have  been  recently  discovered,  but 
whose  dimensions,  and  distances  from  the  sun,  have  not  been 
very  accurately  ascertained.  They  are  generally  called  asteroids. 

Jupiter  is  next  in  order:  this  is  the  largest  of  all  the  planets. 
He  is  about  490  millions  of  miles  from  the  sun,  and  completes 
his  annual  period  in  nearly  12  of  our  years.  He  turns  round  his 
axis  in  about  ten  hours.  He  is  above  1200  times  as  big  as  our 
earth;  his  diameter  is  86,000  miles.     The  respective  proportions 

21.  What  respecting  Venus?  22.  When  does  Venus  become  a  morning, 
and  when  an  evening  star  ?  23.  What  is  said  of  the  Eaith  ?  24.  What  of 
Mars  ?    25.  What  four  small  planets  follow  next  ? 


OF   THE    PLANETS.  S5 

of  the  planets  cannot,  therefore,  you  see,  be  conveniently  deli- 
neated m  a  diagram.     He  is  attended  by  four  moons. 

The  next  planet  is  Saturn,  whose  distance  from  the  sun,  is 
about  900  millions  of  miles ;  his  diurnal  rotation  is  performed  in 
10  hours  and  a  quarter :  his  annual  revolution  is  nearly  30  of 
our  years.  His  diameter  is  79,000  miles.  This  planet  is  sur- 
rounded by  a  luminous  ring,  the  nature  of  which^  astronomers 
are  much  at  a  loss  to  conjecture  :  he  has  seven  moons.  Lastly, 
we  observe  the  planet  Herschel,  discovered  by  Dr.  Herschel,  by 
whom  it  was  named  the  Georgium  Sid  us,  and  which  is  attended 
by  six  moons. 

Caroline,  How  charming  it  must  be  in  the  distant  planets,  to 
see  several  moons  shining  at  the  same  time ;  I  think  I  should 
like  to  be  an  inhabitant  of  Jupiter  or  Saturn. 

Mrs.  B,  Not  long  I  believe.  Consider  what  extreme  cold 
must  prevail  in  a  planet,  situated  as  Saturn  is,  at  nearly  ten 
times  the  distance  at  which  we  are  from  the  sun.  Then  his 
numerous  moons  are  far  from  making  so  splendid  an  appearance 
as  ours ;  for  they  can  reflect  only  the  light  which  they  receive 
from  the  sun ;  and  botli  light,  and  heat,  decrease  in  the  same  ratio 
or  proportion  to  the  distances,  as  gravity.  Can  you  tell  me  now 
how  much  more  light  we  enjoy  than  Saturn  ? 

Caroline.  The  square  of  ten  is  a  hundred ;  therefore,  Saturn 
has  a  hundred  times  less — or  to  answer  your  question  exactly, 
we  have  a  hundred  times  moreli^ht  and  heat,  than  Saturn — this 
certainly  does  not  increase  my  wish  to  become  one  of  the  poor 
wretches  who  inhabit  that  planet. 

Mrs.  B.  May  not  the  inhabitants  of  Mercury,  with  equal 
plausibility,  pity  us  for  the  insupportable  colaness  of  our  situa- 
tion; and  those  of  Jupiter  and  Saturn  for  our  intolerable  heat? 
The  Almighty  power  which  created  these  planets,  and  placed 
them  in  their  several  orbits,  has  no  doubt  peopled  them  with  be- 
ings, whose  bodies  are  adapted  to  the  various  temperatures  and 
elements,  in  which  thev  are  situated.  If  we  judge  from  the 
analogy  of  our  own  eartli,  or  from  that  of  the  great  and  univer- 
sal beneficence  of  Providence,  we  must  conclude  this  to  be  the 
case.  ^ 

Caroline.     Are  not  comets,  in  some  respects  similar  to  planets  ? 

Mrs.  B.  Yes,  they  are ;  for  by  the  reappearance  of  some  of 
them,  at  stated  times,  they  are  known  to  revolve  round  the  sun; 
but  in  orbits  so  extremely  eccentric,  that  they  disappear  for  a 
great  number  of  years.  If  they  are  inhabited,  it  must  be  by  a 
species  of  beings  very  different,  not  only  from  the  inhabitants  of 

26.  What  is  said  of  Jupiter?  27.  What  of  Saturn?  28.  What  of  Her- 
schel ?  29.  Why  do  we  conclude  that  the  moons  of  Saturn  afford  less  light 
than  ours  ?  30.  In  what  proportion  will  the  light  and  heat  at  Saturn  be  di- 
minished, and  why  ? 

H 


86  OF   THE    PLANETS. 

this,  but  from  those  of  any  of  the  other  planets,  as  they  must 
experience  the  greatest  vicissitudes  of  heat  and  cold ;  one  part 
of  their  orbit  being  so  near  the  sun,  that  their  heat,  when  there, 
is  computed  to  be  greater  than  that  of  red-hot  iron ;  in  this  part 
of  its  orbit,  the  comet  emits  a  luminous  vapour,  called  the  tail, 
which  it  gradually  loses  as  it  recedes  from  the  sun;  and  the 
comet  itself  totally  disappears  from  our  siffht,  in  the  more  dis- 
tant parts  of  its  orbit,  which  extends  considerably  beyond  that 
of  the  furthest  planet. 

The  number  of  comets  belonging  to  our  system  cannot  be  as- 
certained, as  some  of  them  are  several  centuries  before  they  make 
their  reappearance.  The  number  that  are  known  by  their  regu- 
lar reappearance  is,  I  believe,  only  three,  although  their  whole 
number  is  very  considerable. 

Emily.    Pray,  Mrs.  B.,  what  are  the  constellations  ? 

Mrs,  B.  They  are  the  fixed  stars;  which  the  ancients,  in  or- 
der to  recognise  them,  formed  into  groups,  and  gave  the  names 
of  the  figures,  which  you  find  delineated  on  the  celestial  globe. 
In  order  to  show  their  proper  situations  in  the  heavens,  they 
should  be  painted  on  the  internal  surface  of  a  hollow  sphere, 
from  the  centre  of  which  you  should  view  them ;  you  would  then 
behold  tiiem  as  tliey  appear  to  be  situated  in  the  heavens.  The 
twelve  constellations,  called  the  signs  of  the  zodiac,  are  those 
wliich  are  so  situated,  that  the  earth,  in  its  annual  revolution, 
passes  directly  between  them,  and  the  sun.  Their  names  are 
Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Libra,  Scorpio,  Sa- 
gittarius, Capricornus,  Aquarius,  Pisces ;  the  whole  occupying  a 
complete  circle,  or  broad  belt,  in  the  heavens,  called  the  zodiac, 
(plate  8.  fig.  1.)  Hence,  a  right  line  drawn  from  the  earth,  and 
passing  through  the  sun,  would  reach  one  of  these  constellations, 
and  the  sun  is  said  to  be  in  that  constellation  at  which  the  line 
terminates :  thus,  when  the  earth  is  at  A,  the  sun  would  appear 
to  be  in  the  constellation  or  sign  Aries ;  when  the  earth  is  at  B, 
the  sun  would  appear  in  Cancer;  when  the  earth  was  at  C,  the 
sun  would  be  in  Libra;  and  when  the  earth  was  at  D,  the  sun 
would  be  in  Capricorn.  You  are  aware  that  it  is  the  real  motion 
of  the  earth  in  its  orbit,  which  gives  to  the  sun  this  apparent 
motion  through  the  signs.  This  circle,  in  which  the  sun  thus 
appears  to  move,  and  which  passes  through  the  middle  of  the 
zodiac,  is  called  the  ecliptic. 

Caroline.  But  many  of  the  stars  in  these  constellations  ap- 
pear beyond  the  zodiac. 

31.  What  do  the  comets  resemble,  and  what  is  remarkable  in  their  orbits? 
.32.  What  is  said  of  the  number  of  comets  ?     33.  What  is  a  constellation  ? 

34.  How  are  the  twelve  constellations,  or  signs,  called  the  zodiac,  situated  ? 

35.  Name  them.  36.  What  is  meant  by  the  sun  being  in  a  sign  ?  37.  What 
cause*  the  apparent  change  of  the  sun's  place  i* 


|: 


*   !*» 


mp 


OF   THE    PLANETS. 


sr 


Mrs.  B.  We  have  no  means  of  ascertaining  the  distance  of 
the  fixed  stai's.  When,  therefore,  they  are  said  to  be  in  the 
zodiac,  it  is  merely  implied  that  they  are  situated  in  that  direc- 
tion, and  that  they  shme  upon  us  through  that  portion  of  the 
heavens,  which  we  call  the  zodiac. 

Emily.  But  are  not  those  large  bright  stars,  which  are  called 
stars  of  the  first  magnitude,  nearer  to  us,  than  those  small  ones 
which  we  can  scarcely  discern? 

Mrs.  B,  It  may  be  so ;  or  the  difference  of  size  and  brillian- 
cy of  the  stars(may  proceed  from  their  difference  of  dimensions; 
this  is  a  point  which  astronomers  are  not  enabled  to  determine. 
Considering  them  as  suns,  I  see  no  reason  why  different  suns 
should  not  vary  in  dimensions,  as  well  as  tlie  planets  belonging 
to  them. 

Emily.  What  a  wonderful  and  beautiful  system  this  is,  and 
how  astonishing  to  think  that  every  fixed  star  may  probably  be 
attended  by  a  similar  train  of  planets ! 

Caroline.  You  will  accuse  me  of  being  very  incredulous,  but 
I  cannot  help  still  entertaining  some  doubts,  and  fearing  that 
there  is  more  beauty  than  truth  in  this  system.  It  certainly  may 
be  so;  but  there  does  not  appear  to  me  to  be  sufficient  evidence 
to  prove  it.  It  seems  so  plain  and  obvious  that  the  earth  is  mo- 
tionless, and  that  the  sun  and  stars  revolve  round  it ; — ^your  solar 
system,  you  must  allow,  is  directly  in  opposition  to  the  evidence 
of  our  senses. 

Mrs.  B.  Our  senses  so  often  mislead  us,  that  we  should  not 
place  implicit  reliance  upon  them. 

Caroline.  On  what  then  can  we  rely,  for  do  we  not  receive 
all  our  ideas  through  the  medium  of  our  senses  ? 

Mrs.  B.  It  is  true  that  they  are  our  primary  source  of  know- 
ledge ;  but  the  mind  has  the  power  of  reflecting,  judging,  and 
deciding  upon  the  ideas  received  by  the  organs  of  sense.  This 
faculty,  which  we  call  reason,  has  frequently  proved  to  us,  that 
our  senses  are  liable  to  err.  If  you  have  ever  skilled  on  the  water, 
with  a  very  steady  breeze,  you  must  have  seen  the  houses,  trees, 
and  every  object  on  the  shore  move,  while  you  were  sailing. 

Caroline.  I  remember  thinking  so,  when  I  was  very  young ; 
but  I  now  know  that  their  motion  is  only  apparent.  It  is  true 
that  my  reason,  in  this  case,  corrects  the  error  of  my  sight. 

Mrs.  B.  It  teaches  you,  that  the  apparent  motion  of  the  ob- 
jects on  shore,  proceeds  from  your  bemg  yourself  moving,  and 
that  you  are  not  sensible  of  your  own  motion,  because  you  meet 
with  no  resistance.  It  is  only  when  some  obstacle  impedes  our 
motion,  that  we  are  conscious  of  moving;  and  if  you  were  to 

38.  The  stars  appear  of  different  magnitudes,  by  what  may  this  be  caused? 
39.  We  are  not  sensible  of  the  motion  of  the  earth ;  what  fact  is  mentioned  to 
illustrate  this  point?     40.  What  does  this  teach  us? 


88  OF  THE   PLANETS. 

close  your  eyes  when  you  were  sailing  on  calm  water,  with  a 
steady  wind,  you  would  not  perceive  that  you  moved,  for  you 
could  not  feel  it,  and  you  could  see  it  only  by  observing  the 
change  of  place  of  the  objects  on  shore.  So  it  is  with  the  motion 
of  the  earth :  every  thing  on  its  surface,  and  the  air  that  surrounds 
it,  accompanies  it  in  its  revolution ;  it  meets  with  no  resistance : 
therefore,  like  the  crew  of  a  vessel  sailing  with  a  fair  wind,  in  a 
tjalm  sea,  we  are  insensible  of  our  motion. 

Caroline.  But  the  principal  reason  why  the  crew  of  a  vessel 
in  a  calm  sea  do  not  perceive  their  motion,  is,  because  they  move 
exceedingly  slow,  while  the  earth,  you  say,  revolves  with  great 
velocity. 

Mrs.  B.  It  is  not  because  they  move  slowly,  but  because  they 
move  steadily,  and  meet  with  no  irregular  resistances,  that  the 
crew  of  a  vessel  do  not  perceive  their  motion ;  for  they  would  be 
equally  insensible  to  it,  with  the  strongest  wind,  provided  it 
it  were  steady,  that  they  sailed  with  it,  and  that  it  did  not  ad- 
tate  the  water;  but  this  last  condition, you  know,  is  not  possible, 
for  the  wind  will  always  produce  waves  which  offer  more  or  less 
resistance  to  the  vessel,  and  then  the  motion  becomes  sensible, 
because  it  is  unequal. 

Caroline.  But,  granting  this,  the  crew  of  a  vessel  have  a 
proof  of  their  motion,  which  the  inhabitants  of  the  earth  cannot 
nave, — the  apparent  motion  of  the  objects  on  shore,  or  their  hav- 
ing passed  from  one  place  to  another. 

Mrs.  B.  Have  we  not  a  similar  proof  of  the  earth's  motion, 
in  the  apparent  motion  of  the  sun  and  stars  ?  Imagine  the  earth 
to  be  sailing  round  its  axis,  and  successively  passing  by  every 
star,  which,  like  the  objects  on  land,  we  suppose  to  be  moving 
instead  of  ourselves.  I  have  heard  it  observed  by  an  aerial  tra- 
veller in  a  balloon,  that  the  earth  appears  to  sink  beneath  the 
balloon,  instead  of  the  balloon  rising  above  the  earth. 

It  is  a  law  which  we  discover  throughout  nature,  and  worthy 
of  its  great  Author,  that  all  its  purposes  are  accomplished  by  the 
most  simple  means ;  and  what  reason  have  we  to  suppose  this 
law  infringed,  in  order  that  we  may  remain  at  rest,  while  the 
sun  and  stars  move  round  us;  their  regular  motions,  which  are 
explained  by  the  laws  of  attraction,  on  the  first  supposition,  would 
be  unintelligible  on  the  last,  and  the  order  and  harmony  of  the 
universe  be  destroyed.  Think  what  an  immense  circuit  the  sun 
and  stars  would  make  daily,  were  their  apparent  motions,  real. 
We  know  many  of  them,  to  be  bodies  more  considerable  than  our 
earth ;  for  our  eyes  vainly  endeavour  to  persuade  us,  that  they 

41.  Would  the  slowness,  or  the  rapidity  of  the  motion,  if  steady,  produce 
any  sensible  diflference  I     42.  If  we  do  not  feel  the  motion  of  the  earth,  how 

may  we  he  ronriiired  of  its  reality? 


OF   THE    PLANETS.  89 

are  little  brilliants  sparkling  in  the  heavens;  while  science  teaches 
us  that  they  are  immense  spheres,  whose  apparent  dimensions 
are  diminished  bj  distance.  Why  then  should  these  enormous 
globes  daily  traverse  such  a  prodigious  space,  merely  to  prevent 
the  necessity  of  our  earth's  revolvmg  on  its  axis  ? 

Caroline.  I  think  I  must  now  be  convinced.  But  you  will,  I 
hope,  allow  me  a  little  time  to  familiarise  to  myself,  an  idea  io 
different  from  that  which  I  have  been  accustomed  to  entertain. 
And  pray,  at  what  rate  do  we  move? 

Mrs.  B.  The  motion  produced  by  the  revolution  of  the  earth 
on  its  axis,  is  abou^  seventeen  miles  a  minute,  to  an  inhabitant 
on  the  equator. 

Emily.  But  does  not  every  part  of  the  earth  move  with  the 
same  velocity  ? 

Mrs.  B.  A  moment's  reflection  would  convince  you  of  the 
contrary :  a  person  at  the  equator  must  move  quicker  than  one 
situated  near  the  poles,  since  they  both  perform  a  revolution  in 
24  hours. 

Emily.  True,  the  equator  is  farthest  from  the  axis  of  motion. 
But  in  the  earth's  revolution  round  the  sun,  every  part  must 
move  with  equal  velocity  ? 

Mrs.  B.     Yes,  about  a  thousand  miles  a  minute. 

Caroline.  How  astonishing ! — and  that  it  should  be  possible 
for  us  to  be  insensible  of  such  a  rapid  motion.  You  would  not 
tell  me  this  sooner,  Mrs.  B.,  for  fear  of  increasing  my  incredulity. 

Before  the  time  of  Newton,  was  not  the  earth  supposed  to  be 
in  the  centre  of  the  system,  and  the  sun,  moon,  and  stars  to 
revolve  round  it  ? 

Mrs.  B.  This  was  the  system  of  Ptolemy,  in  ancient  times; 
but  as  long  ago  as  the  beginning  of  the  sixteenth  century  it  was 
generally  discarded,  and  the  solar  system,  such  as  I  have  shown 
you,  was  established  by  the  celebrated  astronomer  Copernicus, 
and  is  hence  called  the  Copernican  system.  But  the  theory  of 
gravitation,  the  source  from  which  this  beautiful  and  harmonious 
arrangement  flows,  we  owe  to  the  powerful  genius  of  Newton, 
who  lived  at  a  much  later  period,  and  who  demonstrated  its 
truth. 

Emily.  It  appears,  indeed,  far  less  difficult  to  trace  by  obser- 
vation me  motion  of  the  planets,  than  to  divine  by  what  power 

43.  Were  we  to  deny  the  motion  of  the  earth  upon  its  axis,  what  must  we 
admit  respecting  the  heavenly  bodies  ?  44.  What  distance  is  an  inhabitant  on 
the  equator  carried  in  a  minute  by  the  diurnal  motion  of  the  earth  ?  45.  Why 
is  not  tlie  velocity  every  where  equally  great?  46.  What  distance  does  the 
earth  travel  in  a  minute,  in  its  revolution  round  the  sun  ?  47.  What  was 
formerly  supposed  respecting  the  motion  of  all  the  heavenly  bodies  ?  48.  What 
do  we  mean  by  the  Copernican  system,  and  what  is  said  respecting  Coper- 
nicus and  Newton  ? 

H2 


90  «F   THE   PLANETS. 

they  are  impelled  and  guided.  I  wonder  how  the  idea  of  gra- 
vitation could  first  have  occurred  to  sir  Isaac  Newton  ? 

Mrs,  B.  It  is  said  to  have  been  occasioned  bja  circumstance 
from  which  one  should  little  have  expected  so  grand  a  theory  to 
have  arisen. 

During  the  prevalence  of  the  plague  in  the  year  1665,  Newton 
retired  into  the  country  to  avoia  the  contagion :  when  sitting  one 
day  in  an  orchard,  he  observed  an  apple  fall  from  a  tree,  and 
was  led  to  consider  what  could  be  the  cause  which  brought  it  to 
the  ground. 

Caroline,  If  I  dared  to  confess  it,  Mrs.  B.,  I  should  say  that 
such  an  inquiry  indicated  rather  a  deficiency  than  a  superiority 
of  intellect.  I  do  not  understand  how  any  one  can  wonder  at 
what  is  so  natural  and  so  common. 

Mrs,  B.  It  is  the  mark  of  superior  genius  to  find  matter  for 
wonder,  observation,  and  research,  in  circumstances  which,  to 
the  ordinary  mind,  appear  trivial,  because  they  are  common;  and 
with  which  they  are  satisfied,  because  they  are  natural;  without 
reflecting  that  nature  is  our  grand  field  of  observation,  that  with- 
in it,  is  contained  our  whole  store  of  knowledge ;  in  a  word,  that 
to  study  the  works  of  nature,  is  to  learn  to  appreciate  and  ad- 
mire the  wisdom  of  God.  Thus,  it  was  the  simple  circumstance 
of  the  fall  of  an  apple,  which  led  to  the  discovery  of  the  laws 
upon  ^hich  the  Copernican  system  is  founded;  and  whatever 
credit  this  system  had  obtained  before,  it  now  rests  upon  a  basis 
from  which  it  cannot  be  shaken. 

Emily,  This  was  a  most  fortunate  apple,  and  more  worthy 
to  be  commemorated  than  all  those  that  liave  been  sung  by  the 
poets.  The  apple  of  discord  for  which  the  goddesses  contended ; 
the  golden  apples  by  which  Atalanta  won  the  race ;  nay,  even 
the  apple  which  William  Tell  shot  from  the  head  of  his  son,  can- 
not be  compared  to  this ! 

49.  What  circumstance  is  said  to  have  given  rise  to  the  speculations  of 
Newton,  on  the  subject  of  gravitation  ? 


CONVERSATION  VIII. 


ON  THE  EARTH. 

!>P  THE   TERRESTRIAL   GLOBE. — OF  THE   FIGURE  OP  THE   EARTH. — OF 

THE.  PENDULUM. OF  THE  VARIATION   OP   THE  SEASONS,  AND  OP   THE 

LENGTH  OF  DAYS  AND  NIGHTS. — OF  THE  CAUSES  OF  THE  HEAT  OF  SUM- 
MER.— OF  SOLAR,  SIDERIAL,  AND  EaUAL  OR  MEAN  TIME. 

MRS.  B. 

As  the  earth  is  the  planet  in  which  we  are  the  most  particu- 
larly interested,  it  is  my  intention  this  morning,  to  explain  to 
you  the  effects  resulting  from  its  annual,  and  diurnal  motions; 
but  for  this  purpose,  it  will  be  necessary  to  make  you  acquainted 
with  the  terrestrial  globe:  you  have  not  either  of  you,  I  conclude, 
learnt  the  use  of  the  globes  ? 

Caroline.  No;  I  once  indeed,  learnt  by  heart,  the  names  of 
the  lines  marked  on  the  globe,  but  as  I  was  informed  they  were 
only  imaginary  divisions,  they  did  not  appear  to  me  worthy  of 
much  attention,  and  were  soon  forgotten. 

Mrs.  B.  You  supposed,  then,  that  astronomers  had  been  at 
the  trouble  of  inventmg  a  number  of  lines,  to  little  purpose.  It 
will  be  impossible  for  me  to  explain  to  you  the  particular  effects 
of  the  earth's  motion,  without  your  having  acquired  a  knowledge 
of  these  lines :  in  plate  8.  fig.  2.  you  will  find  them  all  deline- 
ated :  and  you  must  learn  them  perfectly,  if  you  wish  to  make 
any  proficiency  in  astronomy. 

Caroline.  I  was  taught  them  at  so  early  an  age,  that  I  could 
not  understand  their  meaning ;  and  I  have  often  heard  you  say, 
that  the  only  use  of  words,  was  to  convey  ideas. 

Mrs.  B.  A  knowledge  of  these  lines,  would  have  conveyed 
some  idea  of  the  manner  in  which  they  were  designed  to  divide 
the  globe  into  parts;  although  the  use  of  these  divisions,  might  at 
that  time,  have  been  too  difficult  for  you  to  understand.  Child- 
hood is  the  season,  when  impressions  on  the  memory  are  most 
strongly  and  most  easily  made :  it  is  the  period  at  which  a  large 
stock  of  terms  should  be  treasured  up,  the  precise  application  of 
which  we  may  learn  when  tlie  understanding  is  more  developed. 
It  is,  I  think,  a  very  mistaken  notion,  that  children  should  be 
taught  such  things  only,  as  they  can  perfectly  understand.  Had 
you  been  early  made  acquainted  with  the  terms  which  relate  to 


92  ON   'mE    EARTH 

figure  and  motion,  how  much  it  would  iiu.;e-  facilitated  your  pro- 
gress in  natural  philosophy.  I  have  been  obliged  to  confine 
myself  to  the  most  common  and  familiar  expressions,  in  explain- 
ing the  laws  of  nature;  although  I  am  convinced  that  appropriate 
and  scientific  terms,  might  have  conveyed  more  precise  and  ac- 
curate ideas,  had  you  been  prepared  to  understand  them. 

Emily.     You  may  depend  upon  our  carefully  learning  the  : 
names  of  these  lines,  Mrs.  B.;  but  before  we  commit  them  to 
memory,  will  you  have  the  goodness  to  explain  them  to  us  ? 

Mrs.  B.  Most  willingly.  This  figure  of  a  globe,  or  sphere, 
represents  the  earth ;  the  line  which  passes  through  its  centre, 
and  on  which  it  turns,  is  called  its  axis,  and  the  two  extremities 
of  the  axis  A  and  B,  are  the  poles,  distinguished  by  the  names 
of  the  north  and  the  south  pole.  The  circle  C  D,  which  divides 
the  globe  into  two  equal  parts  between  the  poles,  and  equally 
distant  from  them,  is  called  the  equator,  or  equinoctial  line;  that 

Eart  of  the  globe  to  the  north  of  the  equator,  is  the  northern 
emisphere;  that  part  to  the  south  of  the  equator,  the  southern 
hemisphere.  The  small  circle  E  F,  which  surrounds  the  north 
pole,  is  called  the  arctic  circle;  that  G  H,  which  surrounds  the 
south  pole,  the  antarctic  circle;  these  are  also  called  polar  circles. 
There  are  two  circles,  intermediate  between  the  polar  circles  and 
the  equator;  that  to  the  north  I  K,  called  the  tropic  of  Cancer; 
that  to  the  south,  L  M,  called  the  tropic  of  Capricorn.  Lastly, 
this  circle,  L  K,  which  divides  the  globe  into  two  equal  parts, 
crossing  the  equator  and  extending  northward  as  far  as  the  tro- 
pic of  Cancer,  and  southward  as  far  as  the  tropic  of  Capricorn,  is 
called  the  ecliptic.  The  delineation  of  the  ecliptic  on  the  ter- 
restrial globe  is  not  without  danger  of  conveying  false  ideas;  for 
the  ecliptic  (as  I  have  before  said)  is  an  imaginary  circle  in  the 
heavens,  passing  through  the  middle  of  the  zodiac,  and  situated 
in  the  plane  of  the  earth's  orbit. 

Caroline.  I  do  not  understand  the  meaning  of  the  plane  of 
the  earth's  orbit. 

Mrs.  B.  A  plane,  is  an  even  flat  surface.  Were  you  to  bend 
a  piece  of  wire,  so  as  to  form  a  hoop,  you  might  then  stretch  a 
piece  of  cloth,  or  paper  over  it,  like  the  head  of  a  drum;  this 
would  form  a  flat  surface,  which  might  be  called  the  plane  of  the 
hoop.  Now  the  orbit  of  the  earth,  is  an  imaginary  circle,  sur- 
rounding the  sun,  and  you  can  readily  imagine  a  plane  extend - 

1.  What  does  the  line  A  B,  (fig.  2  plate  8.)  represent,  and  what  are  its  ex- 
tremities called  ?     2.  What  is  meant  by  the  equator,  and  how  is  it  situated  ? 

3.  There  are  two  hemispheres ;  how  are  they  named   and  distinguished  ? 

4.  What  are  the  circles  near  the  poles  called?  5.  What  do  the  lines  I  K, 
and  L  M,  represent  ?  6.  What  circle  is  in  part  represented  by  the  line  L  Ki* 
7.  Against-  what  mistake  must  you  guard  respecting  this  line  ?  8.  What  is 
meunt  by  a  plane,  and  how  could  one  be  represented  i* 


Plaxeh. 


-^  Ma.  1. 


ON   THE    EARTH.  9^ 

ing  from  one  side  of  this  circle  to  the  other,  filling  up  its  whole 
area :  such  a  plane  would  pass  through  the  centre  of  the  sun,  di- 
viding it  into  hemispheres.  You  may  then  imagine  this  plane 
extended  beyond  the  limits  of  the  earth's  orbit,  on  every  side, 
until  it  reached  those  fixed  stars  which  form  the  signs  of  the  zodiac; 
passing  through  the  middle  of  these  siens,  it  would  give  you  the 
place  of  that  imaginary  circle  in  the  heavens,  call  the  ecliptic; 
which  is  the  sun's  apparent  path.  Let  fig.  1.  plate  9,  represent 
such  a  plane,  S  the  sun,  E  the  earth  with  its  orbit,  and  ABC 
D  the  ecliptic  passing  through  the  middle  of  the  zodiac. 

Emily.  If  the  ecliptic  relates  only  to  the  heavens,  why  is  it 
described  upon  the  terrestrial  globe  } 

Mrs.  B.  It  is  convenient  for  the  demonstration  of  a  variety 
of  problems  in  the  use  of  the  globes;  and  besides,  the  obliquity 
of  this  circle  to  the  equator  is  rendered  more  conspicuous  by  its 
bein»  described  qu  the  same  globe;  and  the  obliquity  of  the  eclip- 
tic shows(}iow  much  the  earth's  axis  is  inclined  to  the  plane  of 
its  orbit.     But  to  return  to  fig.  2.  plate  8. 

The  spaces  between  the  several  parallel  circles  on  the  terres- 
trial globe  are  called  zones:  that  which  is  comprehended  between 
the  tropics  is  distinguished  by  tlie  name  of  the  torrid  zone;  the 
spaces  which  extend  from  the  tropics  to  the  polar  circles,  the 
north  and  south  temperate  zones;  and  the  spaces  contained  with- 
in the  polar  circles,  the  frigid  zones.  By  the  term  zone  is 
meant  a  belt,  or  girdle,  the  frigid  zones,  however,  are  not  belts, 
but  circles,  extending  231  degrees  from  their  centres,  the  poles. 

The  several  lines  winch,  you  observe  to  be  drawn  from  one  pole 
to  the  other,  cutting  the  equator  at  right  angles,  are  called  meri- 
dians; the  number  of  these  is  unlimited,  as  a  line  passing  through 
any  place,  directly  to  the  poles,  is  called  tlie  meridian  of  that 
place.  When  any  one  of  these  meridians  is  exactly  opposite  to 
the  sun,  it  is  mid-day,  or  twelve  o'clock  in  the  day,  at  all  the 
places  situated  any  where  on  that  meridian;  and,  at  the  places 
situated  on  the  opposite  meridian,  it  is  consequently  midnight. 

Emily.  To  places  situated  equally  distant  from  these  two 
meridians,  it  must  then  be  six  o'clock. 

9.  Describe  what  is  intended  by  the  plane  of  the  earth's  orbit.  10.  Ex- 
tendings  this  plane  to  the  fixed  stars,  what  circle  would  it  form,  and  among 
what  particular  stars  would  it  be  found?  11.  What  is  fig.  1.  plate  9,  design- 
ed to  represent?  12.  The  ecliptic  does  not  properly  belong  to  the  earth,  for 
what  purpose  then  is  it  described  on  the  terrestrial  globe  ?  13.  What  does 
the  obliquity  of  the  ecliptic  to  the  equator  serve  to  show  ?  14.  Within  what 
limits  do  you  find  the  torrid  zone?  15.  What  two  zones  are  there  between 
the  torrid,  and  the  two  frigid  zones  ?  16.  Where  are  the  frigid  zones  situat- 
ed? 17.  What  is  meant  by  the  term  zone ;  and  are  the  frigid  zones  properly 
so  called?  18.  How  do  meridian  lines  extend,  and  what  is  meant  by  the 
Qieridian  of  a  place  ?  19.  What  is  said  of  the  meridian  to  which  the  sun  ig 
opposite,  and  where  is  it  then  midnight? 


94  ON   THE    EARTH. 

Mrs,  B.  Yes;  if  they  are  to  the  east  of  the  sun's  meridian  it 
is  six  o'clock  in  the  afternoon,  because  they  will  have  previously 
passed  the  sun;  if  to  the  west,  it  is  six  o'clock  in  the  morning, 
and  that  meridian  will  be  proceeding  towards  the  sun. 

Those  circles  which  divide  the  globe  into  two  equal  parts, 
such  as  the  equator  and  the  ecliptic,  are  called  greater  circles; 
to  distinguish  them  from  those  wnich  divide  it  into  two  unequal 
parts,  as  the  tropics,  and  polar  circles,  which  are  called  lesser 
circles.  All  circles,  you  know,  are  imagined  to  be  divided  into 
360  equal  parts,  called  degrees,  and  degrees  are  again  divided 
into  60  equal  parts,  called  minutes.  The  diameter  of  a  circle  is 
a  right  line  drawn  across  it,  and  passing  through  its  centre:  were 
you,  for  instance,  to  measure  across  this  round  table,  that  would 
give  you  its  diameter;  but  were  you  to  measure  all  round  the  edge 
of  it,  you  would  then  obtain  its  circumference. 

Now  Emily,  you  may  tell  me  exactly  how  niany  degrees  are 
contained  in  a  meridian? 

Emily.  A  meridian,  reaching  from  one  pole  to  the  other,  is 
half  a  circle,  and  must  therefore  contain  \  80  degrees. 

Mrs,  B.  Very  well;  and  what  number  of  degrees  are  there 
from  the  equator  to  one  of  the  poles? 

Caroline.  The  equator  being  equally  distant  from-  either  pole, 
that  distance  must  be  half  of  a  meridian,  or  a  quarter  of  the  cir- 
cumference of  a  circle,  and  contain  90  degrees. 

Mrs.  B.  Besides  the  usual  division  of  circles  into  degrees, 
the  ecliptic  is  divided  into  twelve  equal  parts,  called  signs,  which 
bear  the  name  of  the  constellations  through  which  this  circle 
passes  in  the  heavens.  The  degrees  measured  on  the  meridians 
from  the  equator,  either  towards  the  north,  or  towards  the  south, 
are  called  degrees  of  latitude,  of  which  there  may  be  90;  those 
measured  from  east  to  west,  either  on  the  equator,  or  any  of  the 
lesser  circles,  are  called  degrees  of  longitude,  of  whicn  there 
maybe  180;  these  lesser  circles  are  also  called  parallels  of  lati- 
tude. Of  these  parallels  there  may  be  any  number;  a  circle 
drawn  from  east  to  west,  at  any  distance  from  the  equator, 
will  always  be  parallel  to  it,  and  is  therefore  called  a  parallel  of 
latitude. 

20.  What  hour  is  it  then,  at  places  exactly  half  way  between  these  meri- 
dians? 21.  How  are  ^eater  and  lesser  circles  distinguished?  22.  What 
part  of  a  circle  is  a  degree,  an;1  how  are  these  further  divided  ?  23.  What  is 
the  diameter,  and  what  the  circumference  of  a  circle,  and  what  proportion 
do  they  bear  to  each  other?  24.  What  part  of  a  circle  is  a  meridian  ? 
23.  How  many  degrees  are  there  between  the  equator  and  the  poles? 
26.  Into  what  parts,  besides  degrees,  is  the  ecliptic  divided  ?  27.  How  are 
degrees  of  latitude  measured,  and  to  what  number  do  they  extend  ?  28.  On 
what  circles  are  degrees  of  longitude  meastired,  and  to  what  number  do  they 
idxtend  ?     29.  What  is  a  paraUel  of  latitude  ■ 


IPIr 


ON   THE    EARTH.  95 


ISmily.     The  degrees  of  longitude  must  then  vary  in  length, 
according  to  the  dimensions  of  the  circle  on  wliich  they  are  reck- 
I  onedj  those,  for  instance,  at  the  polar  circles,  will  be  considerably 
smaller  than  those  at  the  equator? 

Mrs.  B.  Certainly;  since  the  degrees  of  circles  of  different 
dimensions  do  not  vary  in  number,  tliey  must  necessarily  vary  in 
leng-th.;  The  degrees  of  latitude,  you  may  observe,  never  vary  in 
length;  for  the  meridians  on  which  they  are  reckoned  are  all  of 
the  same  dimensions. 

Emily,     And  of  what  length  is  a  degree  of  latitude.^ 

Mrs.  B.  Sixty  geographical  miles,  which  is  equal  to  69?' 
•  English  statute  miles;  or  about  one-sixth  more  than  a  common 
I  mile. 

'     Emily.     The  degrees  of  longitude  at  the  equator,  must  then 
f'be  of  the  same  dimensions,  with  a  degree  of  latitude. 

Mrs.  B.  They  would,  were  the  earth  a  perfect  sphere;  but  it 
is  not  exactly  such,  being  somewhat  protuberant  about  the 
equator,  and  flattened,  towards  the  poles.  This  form  proceeds 
from  the  superior  action  of  the  centrifugal  power  at  the  equator, 
and  as  this  enlarges  the  circle,  it  must,  in  the  same  proportion, 
increase  the  length  of  the  degrees  of  longitude  measured  on  it. 

Caroline.  I  thought  I  had  understood  the  centrifugal  force 
perfectly,  but  I  do  not  comprehend  its  effects  in  this  instance. 

Mrs.  B.  You  know  that  the  revolution  of  the  earth  on  its  axis, 
must  give  to  every  particle  a  tendency  to  fly  off  from  the  cen- 
tre, that  this  tendency  is  stronger,  or  weaker,  in  proportion  to  the 
velocity  with  which  the  particle  moves;  now  a  particle  situated 
near  to  one  of  the  poles,  makes  one  rotation  in  the  same  space 
of  time  as  a  particle  at  the  equator;  the  latter,  therefore,  having 
a  much  larger  circle  to  describe,  travels  proportionally  faster, 
consequently  the  centrifugal  force  is  much  stronger  at  the  equa- 
tor than  in  the  polar  regions:  it  gradually  decreases  as  you  leave 
the  equator  and  approach  the  poles,  at  which  points  the  cen- 
trifugal force,  entirely  ceases.  Supposing,  therefore,  the  earth 
to  have  been  originally  in  a  fluid  state,  the  particles  in  the  torrid 
zone  would  recede  much  farther  from  the  centre  than  those  in  the 
frigid  zones;  thus  the  polar  regions  would  become  flattened,  and 
those  about  the  equator  elevated. 

As  a  large  portion  of  the  earth  is  covered  with  water,  the 
Creator  gave  to  it  the  form,  denominated  an  oblate  spheroid, 
otherwise  the  polar  regions  would  have  ^en  without  water, 

30.  Degrees  of  longitude  vary  in  length;  what  is  the   cause   of  this? 

31.  What  is  the  length  of  a  degree  of  latitude,  and  why  do  not  these  vary  ? 

32.  What  causes  the  equator  to  be  somewhat  larger  than  a  great  circle  passing 
through  the  poles,  and  what  effect  has  this  on  degrees  of  longitude  measured 
on  the  equator  ?  33.  What  is  the  cause  of  this  ferm  being  given  to  the 
earth  i 


96  ON   THE    EARTH. 

and  those  about  the  equator,  would  have  been  buried  seve; 
miles  below  the  surface  of  the  ocean. 

Caroline.  I  did  not  consider  that  the  particles  in  the  neii 
bourhood  of  the  equator,  move  with  greater  velocity  than  th< 
about  the  poles;  this  was  the  reason  1  could  not  understand  y(Mi. 

Mrs.  B.  You  must  be  careful  to  remember,  that  those  pai  1^ 
of  a  body  which  are  farthest  from  the  centre  of  motion,  must  move 
with  the  greatest  velocity:  tlie  axis  of  the  earth  is  the  centre  of 
its  diurnal  motion,  and  the  equatorial  regions  the  parts  most  dis- 
tant from  the  axis. 

Caroline.  My  head  then  moves  faster  than  my  feet;  and  upon 
the  summit  of  a  mountain,  we  are  carried  round  quicker  than  ivi 
a  valley? 

Mrs.  B.     Certainly;  your  head  is  more  distant  from  the  cen- 
tre of  motion  than  your  feet;  the  mountain-top  than  the  valley;.* 
and  the  more  distant  any  part  of  a  body  is  from  the  centre  of  mo- 
tion, the  larger  is  the  circle  it  will  describe,  and  the  greater 
therefore  must  be  its  velocity.  # 

Emily.  I  have  been  reflecting,  that  if  the  earth  is  not  a  per- 
fect circle— 

Mrs.  B.  A  sphere  you  mean,  my  dear:  a  circle  is  a  round 
line,  every  part  of  which  is  equally  distant  from  the  centre;  a 
sphere  or  globe  is  a  round  body,  the  surface  of  which  is  every 
where  equally  distant  from  the  centre. 

Emily.  If,  then,  the  earth  is  not  a  perfect  sphere,  but  pro- 
minent at  the  equator,  and  depressed  at  the  poles,  would  not  a 
body  weigh  heavier  at  the  equator  than  at  the  poles?  For  the 
earth  beina*  thicker  at  the  equator,  the  attraction  of  gravity  per- 
pendicularly downwards  must  be  stronger. 

Mrs.  B.  Your  reasoning  has  some  plausibility,  but  I  am 
sorry  to  be  obliged  to  add,  that  it  is  quite  erroneous;  for  the 
nearer  any  part  of  the  surface  of  a  body  is  to  the  centre  of  attrac- 
tion, the  more  strongly  it  is  attracted;  because  it  is  then  nearest 
to  the  whole  mass  of  attracting  matter.  In  regard  to  its  effects, 
you  might  consider  the  whole  power  of  gravity,  as  placed  at  the 
centre  of  attraction. 

Emily.  But  were  you  to  penetrate  deep  into  the  earth, 
would  gravity  increase  as  you  approached  the  centre? 

Mrs.  B.  Certainly  not;  I  am  referring  only  to  any  situation 
on  the  surface  of  the  earth.  Were  you  to  penetrate  into  the  inte- 
rior, the  attraction  of  the  parts  above  you,  would  counteract  that 
of  the  parts  beneath  you,  and  consequently  diminish  the  power 
of  gravity  in  proportion  as  you  approach  the  centre;  and  if  you 

34.  What  would  have  been  a  consequence  of  the  centrifugal  force,  had 
the  earth  been  a  perfect  sphere  ?  35.  A  body  situated  at  the  poles,  is  at- 
tracted more  forcibly  than  if  placed  at  the  equator,  what  is  the  reason  .' 


»F    THE    FLANETS.  85 

of  the  planets  cannot,  therefore,  you  see,  be  conveniently  deli- 
neated m  a  diagram.     He  is  attended  by  four  moons. 

The  next  phmet  is  Saturn,  whose  distance  from  the  sun,  is 
about  900  millions  of  miles ;  his  diurnal  rotation  is  performed  in 
10  hours  and  a  quarter:  his  annual  revolution  is  nearly*  30  of 
our  years.  His  diameter  is  79,000  miles.  This  planet  is  sur- 
rounded by  a  luminous  ring,  the  nature  of  which,  astronomers 
are  much  at  a  loss  to  conjecture :  he  has  seven  moons.  Lastly, 
we  observe  the  planet  Herschel,  discovered  by  Dr.  Herschel,  by 
whom  it  was  named  the  Georgium  Sid  us,  and  which  is  attended 
by  six  moons. 

Carolme.  How  charming  it  must  be  in  the  distant  planets,  to 
see  several  moons  shining  at  the  same  time;  I  think  I  should 
like  to  be  an  inhabitant  of  Jupiter  or  Saturn. 

3Irs.  B.  Not  long  I  believe.  Consider  what  extreme  cold 
must  prevail  in  a  planet,  situated  as  Saturn  is,  at  nearly  ten 
times  the  distance  at  which  we  are  from  the  sun.  Then  his 
numerous  moons  are  far  from  making  so  splendid  an  appearance 
as  ours ;  for  they  can  reflect  only  the  light  which  they  receive 
from  the  sun  ;  and  both  light,  and  heat,  decrease  in  the  same  ratio 
or  proportion  to  the  distances,  as  gravity. .  Can  you  tell  me  now 
how  much  more  light  we  enjoy  than  Saturn  } 

Caroline.  The  square  oi  ten  is  a  hundred ;  therefore,  Saturn 
has  a  hundred  times  less — or  to  answer  your  question  exactly, 
we  have  a  hundred  times  more  light  and  heat,  than  Saturn — ^this 
certainly  does  not  increase  my  wish  to  become  one  of  the  poor 
wretches  who  inhabit  that  planet. 

Mrs.  B.  May  not  the  inhabitants  of  Mercury,  with  equal 
plausibility,  pity  us  for  the  insupportable  coldness  of  our  situa- 
tion ;  and  those  of  Jupiter  and  Saturn  for  our  intolerable  heat  ? 
The  Almighty  power  which  created  these  planets,  and  placed 
them  in  their  several  orbits,  has  no  doubt  peopled  them  with  be- 
ings, whose  bodies  are  adapted  to  the  various  temperatures  and 
elements,  in  which  they  are  situated.  If  we  judge  from  the 
analogy  of  our  own  earth,  or  from  that  of  the  great  and  univer- 
sal beneficence  of  Providence,  we  must  conclude  this  to  be  the 
case. 

Carolme.    Are  not  comets,  in  some  respects  similar  to  planets  ? 

Airs.  B.  Yes,  they  are ;  for  by  the  reappearance  of  some  of 
them,  at  stated  times,  they  are  known  to  revolve  round  the  sun ; 
but  in  orbits  so  extremely  eccentric,  that  they  disappear  for  a 
great  number  of  years.  If  they  are  inhabited,  it  must  be  by  a 
species  of  beings  very  different,  not  only  from  the  inhabitants  of 

26.  What  is  said  of  Jupiter?     27.  What  of  Saturn?     28.  What  of  Her- 
schel ?     29.  Why  do  we  conclude  that  the  moons  of  Saturn  afford  less  light 
than  ours  ?     30.  In  what  proportion  will  the  light  and  heat  at  Saturn  be  di- 
minished, and  why  ? 
H 


86  OF   THE    PLANETS. 

this,  but  from  those  of  any  of  the  other  planets,  as  they  must 
experience  the  greatest  vicissitudes  of  heat  and  cold ;  one  part 
of  their  orbit  being  so  near  the  sun,  that  their  heat,  when  there, 
is  computed  to  be  greater  than  that  of  red-hot  iron ;  in  this  part 
of  its  orbit,  the  comet  emits  a  luminous  vapour,  called  the  tail, 
which  it  gradually  loses  as  it  recedes  from  the  sun;  and  the 
comet  itself  totally  disap|)ears  from  our  si^ht,  in  the  more  dis- 
tant parts  of  its  orbit,  which  extends  considerably  beyond  that 
of  the  furthest  planet. 

The  number  of  comets  belonging  to  our  system  cannot  be  as- 
certained, as  some  of  them  are  several  centuries  before  they  make 
their  reappearance.  The  number  that  are  known  by  their  regu- 
lar reappearance  is,  I  believe,  only  three,  although  their  whole 
number  is  very  considerable. 

Emily.     Pray,  Mrs.  B.,  what  are  the  constellations.^ 

Mrs.  B.  They  are  the  fixed  stars;  which  the  ancients,  in  or- 
der to  recognise  them,  formed  into  groups,  and  gave  the  names 
of  the  figures,  which  you  find  delineated  on  the  celestial  globe. 
In  order  to  show  their  proper  situations  in  the  heavens,  they 
should  be  painted  on  the  internal  surface  of  a  hollow  sphere, 
from  the  centre  of  which  you  should  view  them ;  you  would  then 
behold  them  as  they  appear  to  be  situated  in  the  heavens.  The 
twelve  constellations,  called  the  signs  of  the  zodiac,  are  those 
which  are  so  situated,  that  the  earth,  in  its  annual  revolution, 
passes  directly  between  them,  and  the  sun.'»  Their  names  are 
Aries,  Taurus,  Gemini,  Cancer,  Leo,  Virgo,  Libra,  Scorpio,  Sa- 
gittarius, Capricornus,  Aquarius,  Pisces ;  the  whole  occupying  a 
complete  circle,  or  broad  belt,  in  the  heavens,  called  the  zodiac, 
(plate  8.  fig,  1.)  Hence,  a  right  line  drawn  from  the  earth,  and 
passing  through  \\\(^  sun,  would  reach  one  of  these  constellations, 
and  the  sun  is  said  to  be  in  that  constellation  at  which  the  line 
terminates :  thus,  when  the  earth  is  at  A,  the  sun  would  appear 
to  be  in  the  constellation  or  sign  Aries ;  when  the  earth  is  at  B, 
the  sun  would  appear  in  Cancer ;  when  the  earth  was  at  C,  the 
sun  would  be  in  Libra;  and  when  the  earth  was  at  D,  the  sun 
would  be  in  Capricorn.  You  are  aware  that  it  is  the  real  motion 
of  the  earth  in  its  orbit,  which  gives  to  the  sun  this  apparent 
motion  through  the  signs.  This  circle,  in  which  the  sun  thus 
appears  to  move,  and  which  passes  through  the  middle  of  the 
zodiac,  is  called  the  ecliptic. 

Caroline,  But  many  of  the  stars  in  these  constellations  ap- 
pear beyond  the  zodiac. 

31.  What  Jo  the  comets  resemble,  and  what  is  remarkable  in  their  orbits  f 
32.  What  is  said  of  the  number  of  comets  ?     33.  What  is  a  constellation  ? 

34.  How  are  the  twelve  constellations,  or  signs,  called  the  zodiac,  situated  ? 

35.  Name  tliem.  36.  What  is  mfeant  by  the  sun  being  in  a  sign  ?  37.  Whdt 
cause*  the  apparent  change  of  the  sun's  place .'' 


m 


OF   THE    PLANETS.  87 

Mrs.  B.  We  have  no  means  of  ascertaining  the  distance  of 
the  fixed  stars.  When,  therefore,  they  are  said  to  be  in  the 
zodiac,  it  is  merely  implied  that  they  are  situated  in  that  direc- 
tion, and  that  they  shme  upon  us  through  that  portion  of  the 
heavens,  which  we  call  the  zodiac. 

Emily,  But  are  not  those  large  bright  stars,  which  are  called 
stars  of  the  first  magnitude,  nearer  to  us,  than  those  small  ones 
which  we  can  scarcely  discern.^ 

Mrs.  B,  It  may  be  so ;  or  the  difference  of  size  and  brillian- 
cy of  the  starsCinay  proceed  from  their  difference  of  dimensions; 
tliis  is  a  point  which  astronomers  are  not  enabled  to  determine, 
ronsidering  them  as  suns,  I  see  no  reason  why  different  suns 

oidd  not  vary  in  dimensions,  as  well  as  the  planets  belonging 

them.  * 

Emily,  What  a  wonderful  and  beautiful  system  this  is,  and 
}iow  astonishing  to  think  that  every  fixed  star  may  probably  be 
attended  by  a  similar  train  of  planets! 

Caroline.  You  will  accuse  me  of  being  very  incredulous,  but 
I  cannot  help  still  entertaining  some  doubts,  and  fearing  that 
there  is  more  beauty  than  truth  in  this  system.  It  certainly  may 
be  so;  but  there  does  not  appear  to  me  to  be  sufficient  evidence 
to  prove  it.  It  seems  so  plam  and  obvious  that  the  earth  is  mo- 
tionless, and  that  the  sun  and  stars  revolve  round  it ; — your  solar 
system,  you  must  allow,  is  directly  in  opposition  to  the  evidence 
qf  our  senses.  « 

Mrs.  B.  Our  senses  so  often  mislead  us,  that  we  should  not 
place  implicit  reliance  upon  them. 

Caroline.  On  what  then  can  we  rely,  for  do  we  not  receive 
all  our  ideas  through  the  medium  of  our  senses  ? 

Mrs.  B.  It  is  true  that  they  are  our  primary  source  of  knov- 
led^e;  but  the  mind  has  the  power  of  reflecting,  judging,  j;])' 
deciding  upon  the  ideas  received  by  the  organs  of  sense.  This 
faculty,  which  we  call  reason,  has  frequently  proved  to  us,  that 
our  senses  are  liable  to  err.  If  3^ou  have  ever  sailed  on  the  water, 
with  a  very  steady  breeze,  you  must  have  seen  the  houses,  trees, 
and  every  object  on  the  shore  move,  while  you  were  sailing. 

Caroline.  I  remember  tliinking  so,  when  I  was  very  young ; 
but  I  now  know  that  their  motion  is  only  apparent.  It  is  true 
that  my  reason,  in  this  case,  corrects  the  error  of  my  sight. 

Mrs.  B.  It  teaches  you,  that  the  apparent  motion  of  the  ob- 
jects on  shore,  proceeds  from  your  bemg  yourself  moving,  and 
that  you  are  not  sensible  of  your  own  motion,  because  you  meet 
vvid^no  resistance.  It  is  only  when  some  obstacle  impedes  our 
mOTron,  that  we  are  conscious  of  moving;  and  if  you  were  to 
38.  The  stars  appear  of  different  magnitudes,  by  what  may  this  be  caused  ? 
39.  We  are  not  sensible  of  the  motion  of  the  earth ;  what  fact  is  mentioned  to 
illustrate  this  point  ?     40,  What  does  this  teach  usf 

I 


38  OF   THE    PLANETS. 

close  your  eyes  when  you  were  sailing  on  calm  water,  with  a 
steady  wind,  you  would  not  perceive  that  you  moved,  for  you 
could  not  feel  it,  and  you  could  see  it  only  by  observing  the 
•  hange  of  place  of  the  objects  on  shore.  So  it  is  with  the  motion 
of  the  earth :  every  thing  on  its  surface,  and  the  air  that  surrounds 
It,  accompanies  it  in  its  revolution ;  it  meets  with  no  resistance : 
therefore,  like  the  crew  of  a  vessel  sailing  with  a  fair  wind,  in  a 
calm  sea,  we  are  insensible  of  our  motion. 

Caroline.     But  the  principal  reason  why  the  crew  of  a  vessel 
in  a  calm  sea  do  not  perceive  their  motion,  is,  because  they  move, 
exceedingly  slow,  while  the  earth,  you  say,  revolves  with  great 
velocity. 

Mrs.  B.  It  is  not  because  they  move  slowly,  but  because  they 
move  steadily,  and  meet  with  no  irregular  resistances,  that  the 
crew  of  a  vessel  do  not  perceive  their  motion ;  foirlhey  woukl  be 
equally  insensible  to  it,  with  the  strongest  winTl,  provided  it 
it  were  steady,  that  they  sailed  with  it,  and  that  it  did  not  agi- 
tate the  water"*^  but  this  last  condition,  you  know,  is  not  possible, 
for  the  wind  Will  always  produce  waves  which  offer  more  or  less 
resistance  to  the  vessel,  and  then  the  motion  becomes  sensible, 
because  it  is  unequal. 

Caroline.     But,  granting  this,  the  crew  of  a  vessel  have  a 

f)roof  of  their  motion,  which  the  inhabitants  of  the  earth  cannot 
lave, — the  apparent  motion  of  the  objects  on  shore,  or  their  hav- 
ing passed  from  one  place  to  another. 

Mrs.  B.  Have  w^e  not  a  similar  proof  of  the  earth's  motion, 
On  the  apparent  motion  of  the  sun  and  stars  ?  Imagine  the  earth 
to  be  sailing  round  its  axis,  and  successively  passing  by  every 
star,  which,  like  the  objects  on  land,  we  suppose  to  be  moving 
instead  of  ourselves.  I  have  heard  it  observed  by  an  aerial  tra- 
veller in  a  balloon,  that  the  earth  appears  to  sink  beneath  the 
^  i)alloon,  instead  of  the  balloon  rising  above  the  earth. 

It  is  a  law  which  we  discover  throughout  nature,  and  worthy 
of  its  great  Author,  that  all  its  purposes  are  accomplished  by  the 
nlost  simple  means ;  and  what  reason  have  we  to  suppose  this 
law  infringed,  in  order  that  we  may  remain  at  rest,  while  the 
sun  and  stars  move  round  us;  their  regular  motions,  which  are 
explained  by  the  laws  of  attraction,  on  the  first  supposition,  would 
be  unintelligible  on  the  last,  and  the  order  and  harmony  of  the 
universe  be  destroyed.  Think  what  an  immense  circuit  the  sun 
and  stars  would  make  daily,  were  their  apparent  motions,  real. 
We  know  many  of  them,  to  be  bodies  more  considerable  than  our 
earth;  for  our  eyes  vainly  endeavour  to  persuade  us,  that^ey 

41.  Would  the  slowness,  or  the  rapidity  of  the  motion,  if  steady,  produce 
any  sensible  difference  ?  42.  If  we  do  not  feel  the  motion  of  the  earth,  how 
may  we  be  convinced  of  its  reality  ? 


OF   THE   PLANETS.  89 

are  little  brilliants  sparkling  in  the  heavens;  while  science  teaches 
us  that  they  are  immense  spheres,  whose  apparent  dimensions 
are  diminished  by  distance.  Why  then  should  these  enormous 
globes  daily  traverse  such  a  prodigious  space,  merely  to  prevent 
the  necessity  of  our  earth's  revolving  on  its  axis  ? 

Caroline,  I  think  I  must  now  be  convinced.  But  you  will,  I 
hope,  allow  me  a  little  time  to  familiarise  to  myself,  an  idea  so 
different  from  that  which  I  have  been  accustomed  to  entertain. 
And  pray,  at  what  rate  do  we  move? 

Mrs.  B.  The  motion  produced  by  the  revolution  of  the  earth 
on  its  axis,  is  about  seventeen  miles  a  minute,  to  an  inhabitant 
on  the  equator. 

Emily.  But  does  not  every  part  of  the  earth  move  with  the 
same  velocity  ? 

Mrs.  B.  A  moment's  reflection  would  convince  you  of  the 
contrary :  a  person  at  the  equator  must  move  quicker  than  one 
situated  near  the  poles,  since  they  both  perform  a  revolution  in 
24  hours. 

Emily.  True,  the  equator  is  farthest  from  the  axis  of  motion. 
But  in  the  earth's  revolution  round  the  sun,  every  part  must 
move  with  equal  velocity  } 

Mrs.  B.     Yes,;about  a  thousand  miles  a  minute. 

Caroline.  How  astonishing! — and  that  it  should  be  possible 
for  us  to  be  insensible  of  such  a  rapid  motion.  You  would  not 
tell  me  this  sooner,  Mrs.  B.,  for  fear  of  increasing  my  incredulity. 

Before  the  time  of  Newton,  was  nof^the  earth  supposed  to  be 
in  the  centre  of  the  system,  and  the  sun,  moon,  and  stars  to 
revolve  round  it? 

Mrs.  B.  This  was  the  system  of  Ptolemy,  in  ancient  times ; 
but  as  long  ago  as  the  beginning  of  the  sixteenth  century  it  was 
generally  discarded,  and  the  solar  system,  such  as  I  have  shown 
you,  was  established  by  the  celebrated  astronomer  Copernicus, 
and  is  hence  called  the  Copernican  system.  But  the  theory  of 
gravitation,  the  source  from  which  this  beautiful  and  harmonious 
arrangement  flows,  we  owe  to  the  powerful  genius  of  Newton, 
who  lived  at  a  much  later  period,  and  who  demonstrated  its 
truth, 

Emily.  It  appears,  indeed,  far  less  difficult  to  trace  by  obser- 
vation the  motion  of  the  planets,  than  to  divine  by  what  power 

43.  Were  we  to  deny  the  motion  of  the  earth  upon  its  axis,  what  must  we 
admit  respecting  the  heavenly  bodies  ?  44.  What  distance  is  an  inhabitant  on 
the  equator  carried  in  a  minute  by  the  diurnal  motion  of  the  earth  ?  45.  Why 
is  not  the  velocity  every  where  equally  great?  46.  What  distance  does  the 
earth  travel  in  a  minute,  in  its  revolution  round  the  sun?  47.  What  was 
formerly  supposed  respecting  the  motion  of  all  the  heavenly  bodies?  48.  What 
do  we  mean  by  the  Copernican  system,  and  what  is  said  respecting  Coper- 
nicus and  Newton? 

H2 


90  OF   THE   PLANETS. 

they  are  impelled  and  guided.  I  wonder  how  the  idea  of  gra- 
vitation could  first  have  occurred  to  sir  Isaac  Newton  ? 

Mrs.  B.  It  is  said  to  have  been  occasioned  by  a  circumstance 
from  which  one  should  little  have  expected  so  grand  a  theory  to 
have  arisen. 

During  the  prevalence  of  the  plague  in  the  year  1665,  Newton 
retired  into  the  country  to  avoid  the  contagion:  when  sitting  one 
day  in  an  orchard,  he  observed  an  apple  fall  from  a  tree,  and 
was  led  to  consider  what  could  be  the  cause  which  brought  it  to 
the  ground.' 

Caroline.  If  I  dared  to  confess  it,  Mrs.  B.,  I  should  say  that 
such  an  inquiry  indicated  rather  a  deficiency  than  a  superiority 
of  intellect.  I  do  not  understand  how  any  one  can  wonder  at 
what  is  so  natural  and  so  common. 

Mrs.  B.  It  is  the  mark  of  superior  genius  to  find  matter  for 
wonder,  observation,  and  research,  in  circumstances  which,  to 
the  ordinary  mind,  appear  trivial,  because  they  are  common;  and 
with  which  they  are  satisfied,  because  they  are  natural;  without 
reflecting  that  nature  is  our  grand  field  of  observation,  that  with- 
in it,  is  contained  our  whole  store  of  knowledge ;  in  a  word,  that 
to  study  the  works  of  nature,  is  to  learn  to  appreciate  and  ad- 
mire the  wisdom  of  God.  Thus,  it  was  the  simple  circumstance 
of  the  fall  of  an  apple,  which  led  to  the  discovery  of  the  laws 
upon  which  the  Copernican  system  is  founded;  and  whatever 
credit  this  system  had  obtained  before,  it  now  rests  upon  a  basis 
from  which  it  cannot  be  shaken. 

Emily.  This  was  a  most  fortunate  apple,  and  more  worthy 
to  be  commemorated  than  all  those  that  have  been  sung  by  the 
poets.  The  apple  of  discord  for  which  the  goddesses  contended ; 
tlie  golden  apples  by  which  Atalanta  won  the  race ;  nay,  even 
the  apple  winch  William  Tell  shot  from  the  head  of  his  son,  can- 
not be  compared  to  this ! 

49.  What  circumstance  is  said  to  have  given  rise  to  the  specilations  of 
Newton,  on  the  subject  of  gravitation  ? 


CONVERSATION  VHI. 


ON  THE  EARTH. 

OF  THE  TERRESTRIAL  GLOBE.— OF  THE  FIGURE  OF  THE  EARTH. — OF 
THE  PENDULUM. — OF  THE  VARIATION  OF  THE  SEASONS,  AND  OF  THE 
LENGTH  OF  DAYS  AND  NIGHTS. — OF  THE  CAUSES  OF  THE  HEAT  OF  SUM- 
MER.— OF  SOLAR,  SIDERIAL,  AND  SaUAL  OR  MEAN  TIME. 

MRS.  B. 

As  the  earth  is  the  planet  in  which  we  are  the  most  particu- 
larly interested,  it  is  mj  intention  this  morning,  to  explain  to 
jou  the  effects  resulting  from  its  annual,  and  dmrnal  motions ; 
but  for  this  purpose,  it  will  be  necessary  to  make  you  acquainted 
with  the  terrestrial  globe:  you  have  not  either  of  you,  I  conclude, 
learnt  the  use  of  the  globes  P 

Caroline.  No;  I  once  indeed,  learnt  by  heart,  the  names  of 
the  lines  marked  on  the  globe,  but  as  I  was  informed  they  were 
only  imaginary  divisions,  they  did  not  appear  to  me  worthy  of 
much  attention,  and  were  soon  forgotten. 

Mrs.  B.  You  supposed,  then,  that  astronomers  had  been  at 
the  trouble  of  inventmg  a  number  of  lines,  to  little  purpose.  It 
will  be  impossible  for  me  to  explain  to  you  the  particular  effects 
of  the  earth's  motion,  without  your  having  acquired  a  knowledge 
of  these  lines :  in  plate  8.  fig.  2.  you  wUl  find  them  all  deline- 
ated :  and  you  must  learn  them  perfectly,  if  you  wish  to  make 
any  proficiency  in  astronomy. 

Caroline.  I  was  taught  them  at  so  early  an  age,  that  I  could 
not  understand  their  meaning ;  and  I  have  often  heard  you  say, 
that  the  only  use  of  words,  was  to  convey  ideas. 

Mrs.  B.  A  knowledge  of  these  lines,  would  have  conveyed 
some  idea  of  the  manner  in  which  they  were  designed  to  divide 
the  globe  into  parts;  although  the  use  of  these  divisions,  might  at 
that  time,  have  been  too  difficult  for  you  to  understand.  Child- 
hood is  the  season,  when  impressions  on  the  memory  are  most 
strongly  and  most  easily  made :  it  is  the  period  at  which  a  large 
stock  of  terms  should  be  treasured  up,  the  precise  application  of 
which  we  may  learn  when  the  understanding  is  more  developed. 
It  is,  I  think,  a  very  mistaken  notion,  that  children  should  be 
taught  such  things  only,  as  they  can  perfectly  understand.  Had 
you  been  early  made  acquainted  witn  the  terms  which  relate  to 


92  ON    THE    EARTH. 

figure  and  motion^  how  much  it  would  have  facilitated  your  pro- 
gress  in  natural  philosophy.    I  have  been  obliged  to  confine 
myself  to  the  most  common  and  familiar  expressions,  in  explain- 
ing the  laws  of  nature;  although  I  am  convinced  that  approprir 
and  scientific  terms,  might  have  conveyed  more  precise  and  t 
curate  ideas,  had  you  been  prepared  to  understand  them. 

Emily.     You  may  depend  upon  our  carefully  learning  th'' 
names  of  these  lines,  Mrs.  B.;  but  before  we  commit  the. 
memory,  will  you  have  the  goodness  to  explain  them  to  us  ? 

Mrs.  B.  Most  willingly.  This  figure  of  a  globe,  or  sphere, 
represents  the  earth ;  the  line  which  passes  through  its  centre, 
and  on  which  it  turns,  is  called  its  axis,  and  the  two  extremities 
of  the  axis  A  and  B,  are  the  poles,  distinguished  by  the  names 
of  the  north  and  the  south  pole.  The  circle  C  D,  which  divides 
the  globe  into  two  equal  parts  between  the  fioles,  and  equally 
distant  from  them,  is  called  the  equator, or  equinoctial  line;  that 
part  of  the  globe  to  the  north  of  the  equator,  is  the  northern 
nemisphere ;  that  part  to  the  south  of  the  equator,  the  southern 
hemisphere.  The  small  circle  E  F,  which  surrounds  the  north 
pole,  is  called  the  arctic  circle;  that  G  H,  which  surrounds  the 
south  pole,  the  antarctic  circle;  these  are  also  called  polar  circles. 
There  are  two  circles,  intermediate  between  the  polar  circles  and 
the  equator;  that  to  the  north  I  K,  called  the  tropic  of  Cancer; 
that  to  the  south,  L  M,  called  the  tropic  of  Capricorn.  Lastly, 
this  circle,  L  K,  which  divides  the  globe  into  two  equdl  parts, 
crossing  the  equator  and  extending  northward  as  far  as  the  tro- 
pic of  Cancer,  and  southward  as  far  as  the  tropic  of  Capricorn,  is 
called  the  ecliptic.  The  delineation  of  the  ecliptic  on  the  ter- 
restrial globe  is  not  without  danger  of  conveying  false  ideas;  for 
the  ecliptic  (as  I  have  before  said)  is  an  imaginary  circle  in  the 
heavens,  passing  through  the  middle  of  the  zodiac,  and  situated 
in  the  plane  of  the  eartli's  orbit. 

Caroline.  I  do  not  understand  the  meaning  of  the  plane  of 
the  earth's  orbit. 

Mrs.  B.  A  plane,  is  an  even  flat  surface.  Were  you  to  bend 
a  piece  of  wire,  so  as  to  form  a  hoop,  you  mi»ht  then  stretch  a 
piece  of  cloth,  or  paper  over  it,  like  the  head  of  a  drum;  this 
would  form  a  flat  surface,  which  might  be  called  the  plane  of  the 
hoop.  Now  the  orbit  of  the  «arth,  is  an  imaginary  circle,  sur- 
rounding the  sun,  and  you  can  readily  imagine  a  plane  extend  - 

1.  What  does  the  line  A  B,  (fig.  2  plate  8.)  represent,  and  what  are  its  ex- 
tremities called  ?     2.  What  is  meant  by  the  equator,  and  how  is  it  situated  ? 

3.  There  are  two  hemispheres;  how  are  they  named   and   distinguished? 

4.  What  are  the  circles  near  the  poles  called?  5.  What  do  the  lines  I  K, 
and  L  M,  represent  ?  6.  What  circle  is  in  part  represented  by  the  line  L  K  ? 
7.  Against  what  mistake  must  you  guard  respecting  thialine?  8.  What  is 
meant  by  a  plane,  and  how  could  one  be  represented  ? 


Tlateh. 


++ 
+     + 


ON    THE    EARTH. 


ing  from  one  side  of  this  circle  to  the  other,  filling  up  its  whole 
area :  such  a  plane  would  pass  through  the  centre  of  the  sun,  di- 
viding it  into  hemispheres.  You  may  then  imagine  this  plane 
extended  beyond  the  limits  of  the  earth's  orbit,  on  every  side, 
until  it  reached  those  fixed  stars  which  form  the  signs  of  the  zodiac^ 
passing  through  the  middle  of  these  signs,  it  would  give  you  the 
place  of  that  imaginary  circle  in  the  lieavens,  call  the  ecliptic; 
which  is  the  sun's  apparent  path.  Let  fig.  1.  plate  9,  represent 
Kuch  a  plane,  S  the  sun,  E  the  earth  with  its  orbit,  and  ABC 
I)  the  ecliptic  passing  tlu-ough  the  middle  of  the  zodiac. 

Emily.  If  the  ecliptic  relates  only  to  the  heavens,  why  is  it 
described  upon  the  terrestrial  globe  ? 

Mrs.  B.  (It  is  convenient  for  the  demonstration  of  ^  variety 
of  problems  in  the  use  of  the  globes;  and  besides,  the  obliquity 
of  this  circle  to  the  equator  is  rendered  more  conspicuous  by  its 
being  described  on  the  same  globeftand  the  obliquity  of  the  eclip- 
tic sliows'.how  much  the  earth's  axis  is  inclined  to  the  plane  of 
its  orbit..     But  to  return  to  fig.  2.  plate  8. 

The  spaces  between  the  several  parallel  circles  on  the  terres- 
trial globe  are  called  zones:  that  which  is  comprehended  between 
the  tropics  is  distinguished  by  the  name  of  the  torrid  zone;  the 
spaces  which  extend  from  the  tropics  to  the  polar  circles,  the 
north  and  south  temperate  zones;  and  the  spaces  contained  with- 
in the  polar  circles,  the  frigid  zones.  By  the  term  zone  is 
meant  a  belt,  or  girdle,  the  frigid  zones,  however,  are  not  belts, 
but  circles,  extending  285  degrees  from  their  centres,  the  poles. 

The  several  lines  which,  you  observe  to  be  drawn  from  one  pole 
to  the  other,  cutting  the  equator  at  right  angles,  are  called  meri- 
dians; the  number  of  these  is  unlimited,  as  a  line  passing  through 
any  place,  directly  to  the  poles,  is  called  the  meridian  of  that 
place.  When  any  one  of  these  meridians  is  exactly  opposite  to 
the  sun,  it  is  mid -day,  or  twelve  o'clock  in  the  day,  at  all  the 
places  situated  any  where  on  that  meridian;  and,  at  the  places 
situated  on  the  opposite  meridian,  it  is  consequently  midnight. 

Emily.  To  places  situated  equally  distant  from  these  two 
meridians,  it  must  then  be  six  o'clock. 

9.  Describe  what  is  intended  by  the  plane  of  the  earth's  orbit.  10.  Ex- 
tending this  plane  to  the  fixed  stars,  what  circle  would  it  form,  and  among 
what  particular  stars  would  it  be  found?  11,  What  is  fig.  1.  plate  9,  design- 
ed to  represent  ?  12.  The  ecliptic  does  not  properly  belong  to  the  earth,  for 
what  purpose  then  is  it  described  on  the  terrestrial  globe  ?  13.  What  does 
the  obliquity  of  the  ecliptic  to  the  equator  serve  to  show  ?  14.  Within  what 
limits  do  you  find  the  torrid  zone?  15.  What  two  zon^s  are  there  between 
the  torrid,  and  the  two  frigid  zones  ?  16.  Where  are  the  frigid  zones  situat- 
ed? 17.  What  is  meant  by  the  term  zone  ;  and  are  the  frigid  zones  properly 
so  called?  18.  How  do  meridian  lines  extend,  and  what  is  meant  by  the 
meridian  of  a  place?  19.  What  is  said  of  the  meridian  to  which  the  sun  is 
opposit*?,  and  where  is  it  then  midnight  ? 


94  ox  THE    EARTH. 

Mrs.  B,  Yes;  if  they  are  to  the  east  of  the  sun's  meridian  it 
is  six  o'clock  in  the  afternoon,  because  they  will  have  previously 
passed  the  sun;  if  to  the  west,  it  is  six  o'clock  in  the  morning, 
and  that  meridian  will  be  proceeding  towards  the  sun. 

Those  circles  which  divide  the  globe  into  two  equal  parts, 
such  as  the  equator  and  the  ecliptic,  are  called  greater  circles; 
to  distinguish  them  from  those  which  divide  it  into  two  unequal 
parts,  as  the  tropics,  and  polar  circles,  which  are  called  lesser 
circles.  All  circles,  you  know,  are  imagined  to  be  divided  into 
560  equal  parts,  called  degrees,  and  degrees  are  again  divided 
into  60  equal  parts,  called  minutes.  The  diameter  of  a  circle  is 
a  right  line  drawn  across  it,  and  passing  through  its  centre;  were 
you,  for  instance,  to  measure  across  this  round  table,  that  would 
give  you  its  diameter;  but  were  you  to  measure  all  round  the  edge 
of  it,  you  would  then  obtain  its  circumference. 

Now  Emily,  you  may  tell  me  exactly  how  many  degrees  are 
contained  in  a  meridian? 

Emily.  A  meridian,  reaching  from  one  pole  to  the  other,  is 
half  a  circle,  and  must  therefore  contain  \  80  degrees. 

Mrs.  B.  Very  well;  and  what  number  of  degrees  are  there 
from  the  equator  to  one  of  the  poles? 

Caroline.  The  equator  being  equally  distant  from  either  pole, 
that  distance  must  be  half  of  a  meridian,  or  a  quarter  of  the  cir- 
cumference of  a  circle,  and  contain  90  degrees. 

Mrs.  B.  Besides  the  usual  division  of  circles  into  degrees, 
the  ecliptic  is  divided  into  twelve  equal  parts,  called  signs,  which 
bear  the  name  of  the  constellations  through  which  this  circle 
passes  in  the  heavens.  The  degrees  measured  on  the  meridians 
from  the  equator,  either  towards  the  north,  or  towards  the  souths 
are  called  degrees  of  latitude,  of  which  there  may  be  90;  those 
measured  from  east  to  west,  either  on  the  equator,  or  any  of  the 
lesser  circles,  are  called  degrees  of  longitude,  of  which  there 
maybe  180;  these  lesser  circles  are  also  called  parallels  of  lati- 
tude. Of  these  parallels  there  may  be  any  number;  a  circle 
drawn  from  east  to  west,  at  any  distance  from  the  equator, 
will  always  be  parallel  to  it,  and  is  therefore  called  a  parallel  of 
latitude. 

20.  What  hour  is  it  then,  at  places  exactly  half  T^-ay  between  these  meri- 
dians? 21.  How  are  greater  and  lesser  circles  distinguished?  22.  What 
part  of  a  circle  is  a  degree,  and  how  are  these  further  divided  ?  23.  What  is 
the  diameter,  and  what  the  circumference  of  a  circle,  and  what  proportion 
do  they  bear  to  each  other?  24.  What  part  of  a  circle  is  a  meridian  ? 
26.  How  many  degrees  are  there  between  the  equator  and  the  poles? 
26.  Into  what  parts,  besides  degrees,  is  the  ecliptic  divided  ?  27.  How  are 
degrees  of  latitude  measured,  and  to  what  number  do  they  extend?  28.  On 
what  circles  are  degrees  of  longitude  measured,  and  to  what  number  do  they 
extend  ?     29.   What  is  a  parallel  of  latitude' 


ON   THE    EARTH.  95 

Emily,  The  degrees  of  longitude  must  then  vary  in  length, 
according  to  the  dunensions  of  the  chcle  on  which  they- are  reck- 
oned ;  those,  for  instance,  at  the  polar  circles,  will  be  considerably 
smaller  than  those  at  the  equator? 

Mrs,  B,  Certainly;  'since  the  degrees  of  circles  of  different 
dimensions  do  not  vary  in  number,  they  must  necessarily  vary  in 
leng-th.,  The  degrees  of  latitude,  you  may  observe,  never  vary  in 
length;  for  the  meridians  on  which  they  are  reckoned  are  all  of 
the  same  dimensions. ; 

Emily.     And  of  what  length  is  a  degree  of  latitude? 

Mrs,  B,  1  Sixty  geographical  miles,  which  is  equal  to  69 1 
English  statute  miles;  or  about  one-sixth  more  than  a  common 
laile. 

Emily,  The  degrees  of  longitude  at  the  equator,  must  then 
be  of  the  same  dimensions,  with  a  degree  of  latitude. 

Mrs,  B.  They  would,  were  the  earth  a  perfect  sphere;  but  it 
is  not  exactly  such,  f)eing  somewhat  protuberant  .about  the 
equator,  and  flattened  towards  the  poles.  This  form  proceeds 
from  the  superior  action  of  the  centrifugal  power  at  the  equator, 
and  as  this  enlaro-es  the  circle,  it  must,  in  the  same  proportion, 
increase  the  length  of  the  degrees  of  longitude  measured  on  it. 

Caroline,  I  thought  I  had  understood  the  centrifugal  force 
perfectly,  but  I  do  not  comprehend  its  effects  in  this  instance. 

Mrs,  B,  You  know  that  the  revolution  of  the  earth  on  its  axis, 
must  give  to  every  particle  a  tendency  to  fly  off  from  the  cen- 
tre, that  this  tendency  is  stronger,  or  v/eaker,  in  proportion  to  the 
velocity  widi  which  the  particle  moves;  now  a  particle  situated 
near  to  one  of  the  poles,  makes  one  rotation  in  the  same  space 
of  time  as  a  particle  at  the  equator;  the  latter,  therefore,  having 
a  much  larger  circle  to  describe,  travels  proportionally  faster, 
consequently  the  centrifugal  force  is  much  stronger  at  the  equa- 
tor than  in  the  polar  regions:  it  gradually  decreases  as  you  leave 
the  e(iuator  and  approach  the  poles,  at  which  points  the  cen- 
trifugal force,  entirely  ceases.  Supposing,  therefore,  the  earth 
to  have  been  originally  in  a  fluid  state,  the  particles  in  the  torrid 
zone  would  recede  much  farther  from  the  centre  than  those  in  the 
frigid  zones;  thus  the  polar  regions  v/ould  become  flattened,  and 
those  about  the  equator  elevated. 

As  a  large  portion  of  the  earth  is  covered  with  water,  the 
Creator  gave  to  it  the  form,  denominated  an  oblate  spheroid, 
otherwise  the  polar  regions  would  have  been  without  water, 

~  30.  Degrees  of  longitude   vary  in  length;  what  is  the   cause   of  this? 

31.  What  is  the  length  of  a  degree  of  latitude,  and  why  do  not  these  vary  ? 

32.  What  causes  the  equator  to  be  somewhat  larger  than  a  great  circle  passing 
throu;;h  the  poles,  and  what  effect  has  this  on  degrees  of  longitude  measured 
on  the  equator?  33.  What  is  the  cause  of  this  form  being  given  to  the 
earth  ? 


96  ON   THE    EARTH. 

and  those  about  the  equator,  would  have  been  buried  several  ; 
miles  below  the  surface  of  the  ocean.  | 

Caroline.  I  did  not  consider  that  the  particles  in  the  neigh-  j 
bourhood  of  the  equator,  move  with  greater  velocity  than  those  \ 
about  the  poles;  this  was  the  reason  I  could  not  understand  joii.   ^ 

Mrs.  B.     You  must  be  careful  to  remember,  that  those  pa 
of  a  body  which  are  farthest  from  the  centre  of  motion,  must  m< 
with  the  greatest  velocity:  the  axis  of  the  eartli  is  the  centre 
its  diurnal  motion,  and  the  equatorial  regions  the  parts  most  d. 
tant  from  the  axis. 

Caroline.  My  head  then  moves  faster  than  my  feet;  and  upon  ., 
the  summit  of  a  mountain,  we  are  carried  round  quicker  than  in  \ 
a  valley.'' 

Mrs.  B.     Certainly;  your  head  is  more  distant  from  the  Ci 
tre  of  motion  than  your  feet;  the  mountain-top  than  the  vallej  . 
and  the  more  distant  any  part  of  a  body  is  from  the  centre  of  mo-  \ 
tion,  the  larger  is  the  circle  it  will  describe,  and  the  great  - 
therefore  must  be  its  velocity. 

Emily.  I  have  been  reflecting,  that  if  the  earth  is  not  a  per- 
fect circle— 

Mrs.  B.  A  spliere  you  mean,  my  dear:  a  circle  is  a  round  ; 
line,  every  part  of  which  is  equally  distant  from  the  centre;  a  \ 
spliere  or  globe  is  a  round  body,  the  surfiice  of  which  is  every  ' 
where  equally  distant  from  the  centre. 

Emily.  If,  then,  the  earth  is  not  a  perfect  sphere,  but  pro- 
minent at  the  equator,  and  depressed  at  the  poles,  would  not  a 
body  weigh  heavier  at  tlie  equator  than  at  the  poles .^  For  the 
earth  ])eing  thicker  at  the  equator,  the  attraction  of  gravity  per- 
pendicularly downwards  must  be  stronger. 

Mrs.  B.  Your  reasoning  has  some  plausibility,  but  I  am 
sorry  to  be  obliged  to  add,  that  it  is  quite  erroneous;  for  the 
nearer  any  part  of  the  surface  of  a  body  is  to  the  centre  of  attrac- 
tion, the  more  strongly  it  is  attracted;  because  it  is  then  nearest 
to  the  whole  mass  of  attracting  matter.  In  regard  to  its  effects, 
you  might  consider  the  whole  power  of  gravity,  as  placed  at  the 
centre  of  attraction. 

Emily.  But  were  you  to  penetrate  deep  into  the  earth, 
would  gravity  increase  as  you  approached  the  centre? 

Mrs.  B.  Certainly  not .,  I  am  referring  only  to  any  situation 
on  the  surface  of  the  earth.  Were  you  to  penetrate  into  the  inte- 
rior, the  attraction  of  the  parts  above  you,  would  counteract  that 
of  the  parts  beneath  you,  and  consequently  diminish  the  power 
of  gravitj'^  in  proportion  as  you  approach  the  centre;  and  if  you 

34.  What  would  have  been  a  consequence  of  the  centrifua^al  force,  had 
the  earth  been  a  perfect  sphere  ?  35.  A  body  situated  at  the  poles,  is  at- 
toacted  more  forcibly  than  if  placed  at  the  equator,  what  is  the  reason  ? 


ON    THE    EARTH.  9/ 

reached  that  point,  being  equally  attracted  by  the  parts  all  around 
YOU,  the  eftects  of  gravity  would  cease,  and  you  would  be  with- 
(jut  weight. 

Emily.  Bodies,  then,  should  weigh  less  at  the  equator  than 
at  the  poles,  since  they  are  more  distant  from  the  centre  of  gra- 
vity in  the  former  than  in  the  latter  situation? 

Mrs.  B.  And  this  is  really  the  case;  but  the  difference  of 
weight  would  be  scarcely  sensible,  were  it  not  augmented  by  ano- 
ther circumstance. 

Caroline.  And  what  is  this  singular  circumstance,  which 
seems  to  disturb  the  laws  of  nature? 

Mrs.  B.  '  One  that  you  are  well  acquainted  with,  as  conduc- 
ing more  to  the  preservation  than  the  destruction  of  order, — the 
centiifugal  force.  This  we  have  just  observed  to  be  strongest  at 
the  equator;  and  as  it  tends  to  drive  bodies  from  the  centre,  it  is 
necessarily  opposed  to,  and  must  lessen  the  power  of  gravity, 
which  attracts  them  towards  the  centre.  We  accordingly  find 
that  bodies  weigh  lightest  at  the  equator,  where  the  centrifugal 
force  is  greatest;  and  heaviest  at  the  poles,  where  this  power  is 
least:  the  weight  being  diminished  at  the  equator,  by  both  the 
causes  mentioned. 

Caroline.  Has  the  experiment  been  made  in  these  different 
situations  ? 

Mrs.  B.  Louis  XIV.  of  France,  sent  philosophers  both  to  the 
equator,  and  to  Lapland,  for  this  purpose:  the  severity  of  the  cli- 
mate, and  obstruction  from  the  ice,  have  hitherto  rendered  every 
attempt  to  reach  the  pole  abortive;  but  the  difference  of  gravity 
at  the  equator,  and  in  Lapland  is  very  perceptible. 

Caroline.  Yet  I  do  not  comprehend  how  the  difference  of 
weight  could  be  ascertained,  for  it  the  body  under  trial  decreased 
in  weight,  the  weight  which  was  opposed  to  it  in  the  opposite 
scale  must  have  diminished  in  the  same  proportion.  For  in- 
stance, if  a  pound  of  sugar  did  not  weigh  so  heavy  at  the  equator 
as  at  the  poles,  the  leaden  pound  which  served  to  weigh  it,  would 
not  be  so  heavy  either;  therefore  they  would  still  balance  each 
other,  and  the  different  force  of  gravity  could  not  be  ascertained 
by  this  means. 

Mrs.  B.  Your  observation  is  perfectly  just:  the  difference 
of  gravity  in  bodies  situated  at  the  poles,  and  at  the  equator,  can- 
not be  ascertained  by  weighing  them;  a  pendulum  was  therefore 
used  for  that  purpose. 

36.  What  effect  would  be  produced  upon  the  gravity  of  a  body,  were  it 
placed  beneath  the  surface  of  the  earth,  and  what  supposing  it  at  its  centre  ? 
37.  What  two  circumstances  combine,  to  lessen  the  weight  of  a  body  on  the 
equator  ?  38.  Why  could  not  this  be  proved  by  weighing  a  body  at  the  pole», 
and  at  the  equator  ? 

I 


98  eN  THE    EARTH. 

Caroline.  What,  the  pendulum  of  a  clock?  how  could  that 
answer  the  purpose? 

Mrs.  B.  A  pendulum  consists  of  a  line,  or  rod,  to  one  end 
of  which  a  weight  is  attached,  and  by  the  other  end  it  is  suspended 
to  a  fixed  point,  about  which  it  is  made  to  vibrate.  When  not 
in  motion,  a  pendulum,  obeying  the  general  law  of  attraction, 
hanffs  like  a  plumb  line,  perpendicular  to  the  surface  of  the 
earth,  but  if  you  raise  the  pendulum,  gravity  will  bring  it  back  to 
its  perpendicular  position.  It  will,  however,  not  remain  station- 
ary there,  for  the  momentum  it  has  acquired  during  its  descent, 
will  impel  it  onwards,  and  if  unobstructed,  it  will  rise  on  the 
opposite  side  to  an  equal  heightj  from  thence  it  is  brought  back 
by  gravity,  and  is  again  forced  upwards,  by  the  impulse  of  its 
momentum. 

Caroline.  If  so,  the  motion  of  a  pendulum  would  be  perpetual, 
and  I  thought  you  said,  that  there  was  no  perpetual  motion  on 
the  earth. 

Mrs.  B.  The  motion  of  a  pendulum  is  opposed  by  the  resist- 
ance of  the  air  in  which  it  vibrates,  and  by  the  friction  of  the  part 
by  which  it  is  suspended:  were  it  possible  to  remove  these  obsta- 
cles, the  motion  of  a  pendulum  would  be  perpetual,  and  its  vibra- 
tions perfectly  regular  j  each  being  of  equal  distance,  and  per- 
formed in  equal  times. 

Emily.  That  is  the  natural  result  of  the  uniformity  of  the 
power  which  produces  these  vibrations,  for  the  force  of  gravity 
being  always  the  same,  the  velocity  of  the  pendulum  must  conse- 
quently be  uniform. 

Caroline.  No,  Emily,  you  are  mistaken;  the  force  is  not 
every  where  the  same,  and  therefore  the  eftect  will  not  be  so 
either.  I  have  discovered  it,  Mrs.  B.;  since  the  force  of  gravity 
is  less  at  the  equator  than  at  the  poles,  the  vibrations  of  the  pen- 
dulum will  be  slower  at  the  former  place  than  at  the  latter. 

Mrs.  B.  You  are  perfectly  right,  Caroline;  it  was  by  this 
means  that  the  difference  of  gravity  was  discovered,  and  the  true 
figure  of  the  earth  ascertained. 

Emily.  But  how  do  they  contrive  to  regulate  their  time  in 
the  equatorial  and  polar  regions?  for,  since  in  our  part  of  the 
earth  the  pendulum  of  a  clock  vibrates  exactly  once  in  a  second, 
if  it  vibrates  faster  at  the  poles,  and  slower  at  the  equator,  the 
inhabitants  must  regulate  their  clocks  in  a  manner  different 
from  us. 

Mrs.  B.     The  only  alteration  required  is  to  lengthen  the  pen- 

39.  What  is  a  pendulum?  40.  What  causes  it  to  vibrate  ?  41.  Why  are 
not  it«  vibrations  perpetual  ?  42.  Two  pendulums  of  the  same  length,  will 
not,  in  different  latitudes,  perform  their  vibrations  in  equal  times,  what  is  the 
cause  of  this  ?  43.  To  what  use  has  this  property  of  the  pendulum  been  ap- 
j)Ued .' 


ON  THE   EARTH.  9.9 

dulum  in  one  case,  and  to  shorten  it  in  the  other;  for  the  velocity 
of  the  vibrations  of  a  pendulum  depends  on  its  length;  and  when 
it  is  said  that  a  pendulum  vibrates  quicker  at  the  pole  than  at  tlie 
equator,  it  is  supposed  to  be  of  the  same  length.  A  pendulum 
which  vibrates  seconds  in  this  latitude  is  about  39-f  inches 
long.  In  order  to  vibrate  at  the  equator  in  the  same  space  of 
time,  it  must  be  somewhat  shorter;  and  at  the  poles,  it  must 
be  proportionally  lengthened. 

/The  vibrations  of  a  pendulum,  resemble  the  descent  of  a  bod} 
on  an  inclined  plane,  and  are  produced  by  the  same  cause;  now 
you  must  recollect,  that  the  greater  the  perpendicular  height  of 
such  a  plane,  in  proportion  to  its  length,  the  more  rapid  will  be 
the  descent  of  the  body;  a  short  pendulum  ascends  to  a  greater 
height  than  a  larger  one,  in  vibrating  a  given  distance,  and  of 
course  its  descent  must  be  more  rapid.^ 

I  shall  now,  I  think,  be  able  to  explain  to  you  the  cause  of  the 
variation  of  the  seasons,  and  the  difference  in  the  length  of 
the  days  and  nights  in  those  seasons;  both  effects  resulting  from 
the  same  cause. 

In  moving  round  the  sun,  the  axis  of  the  earth  is  not  perpen- 
dicular to  the  plane  of  its  orbit.  Supposing  this  round  table  to 
represent  the  plane  of  the  earth's  orbit,  and  this  little  globe,  the 
earth ;  through  this  I  have  passed  a  wire,  representing  its  axis 
and  poles.  In  moving  round  the  table,  I  do  not  hold  the  wire 
perpendicular  to  it,  but  obliquely. 

Emily.  Yes,  I  understand,  the  earth  does  not  go  round  the 
sun  in  an  upright  position,  its  axis  is  slanting  or  oblique;  and,  it 
of  course,  forms  an  angle  with  a  line  drawn  perpendicular  to  the 
plane  of  the  earth's  orbit. 

Mrs.  B.  All  the  lines,  which  you  learnt  in  your  last  lesson, 
are  delineated  on  this  little  globe;  you  must  consider  the  ecliptic 
as  representing  the  plane  of  the  earth's  orbit;  and  the  equator, 
which  crosses  the  ecliptic  in  two  places,  then  shows  the  degree  of 
obliquity  of  the  axis  of  the  earth;  which  amounts  to  SSj  degrees, 
very  nearly.  The  points  in  which  the  ecliptic  intersects  the 
equator,  are  called  the  equinoctial  points. 

But  I  believe  I  shall  render  the  effects  of  the  obliquity  of  the 
earth's  axis  clearer  to  you,  by  the  revolution  of  the  little  globe 
round  a  candle,  which  shall  represent  the  sun.  (Plate  IX. 
fig.  2.) 

As  I  now  hold  it,  at  A,  you  see  it  in  the  situation  in  which  it  is 

44.  What  change  must  be  made  in  pendulums  situated  at  the  equator  and 
at  the  poles,  to  render  their  vibrations  equal  ?  45.  What  do  the  vibrations 
of  a  pendulum  resemble,  and  why  will  it  vibrate  more  rapidly  if  shortened  ? 
46.  In  the  revolution  of  the  earth  round  the  sun,  what  is  the  position  of  its 
axis  ?  47.  How  much  is  the  axis  of  the  earth  inclined,  and  with  what  line 
does  it  form  this  angle  f     48.  What  is  represented  by  fig.  2,  plate  9  ' 


100 


ON   THE   EARTH. 


in  the  midst  of  summer^  or  what  is  called  the  summer  solstice, 
which  is  on  the  21st  of  June. 

Emily.  You  hold  the  wire  awrj,  I  suppose,  in  order  to  show 
that  the  axis  of  the  earth  is  not  upright? 

Mrs.  B.  Yesj  in  summer, 'the  north  pole  is  inclined  towards 
the  sun.  In  this  season,  therefore,  the  northern  hemisphere  en- 
joys much  more  of  his  rajs  than  the  southern.  The  sun,  you 
see,  now  shines  over  the  whole  of  the  north  frigid  zone;  and  not- 
withstanding the  earth's  diurnal  revolution,  which  I  imitate  by 
twirling  the  ball  on  the  wire,  it  will  continue  to  shine  upon  it  as 
long  as  it  remains  in  this  situation,  whilst  the  south  frigid  zone  is 
at  the  same  time  completely  in  darkness. 

Caroline.  That  is  very  strange;  I  never  before  heard  that 
there  was  constant  day  or  night  in  any  part  of  the  world !  How 
much  happier  the  inhabitants  of  the  north  frigid  zone  must  be 
than  those  of  the  southern;  the  first  enjoy  uninterrupted  day, 
Avhile  the  last  are  involved  in  perpetual  darkness. 

Mrs.  B.  You  judge  with  too  much  precipitation;  examine  a 
little  further,  and  you  will  find,  that  the  two  frigid  zones  share 
an  equal  fate. 

We  shall  now  make  the  earth  set  off  from  its  position  in  the 
summer  solstice,  and  carry  it  round  the  sun;  observe  that  the 
pole  is  always  inclined  in  the  same  direction,  and  points  to  the 
same  spot  in  the  heavens.  There  is  a  fixed  star  situated  near 
that  spot,  which  is  hence  called  the  north  polar  star.  Now  let 
us  stop  the  earth  at  B,  and  examine  it  in  its  present  situation;  it 
has  gone  through  one  quarter  of  its  orbit,  and  is  arrived  at  that 
point  at  which  tiie  ecliptic  cuts,  or  crosses,  the  equator,  and  which 
is  called  the  autumnal  equinox. 

Emily.  The  sun  now  shines  from  one  pole  to  the  other,  just  as 
it  would  constantly  do,  if  the  axis  of  the  earth  were  perpendicu- 
lar to  its  orbit. 

Mrs.  B.  Because  the  inclination  of  the  axis  is  now  neither 
towards  the  sun,  nor  in  the  contrary  direction;  at  this  period  of  the 
year,  the  days  and  nights  are  equal  in  every  part  of  the  earth. 
But  the  next  step  she  takes  in  her  orbit,  you  see,  involves  the 
north  pole  in  darkness,  whilst  it  illumines  that  of  the  south;  this 
change  was  gradually  preparing  as  I  moved  the  earth  from  sum- 
mer to  autumn;  the  arctic  circle,  which  was  at  first  entirely 
illumined,  began  to  have  short  nights,  which  increased  as  the 
earth  approached  the  autumnal  equinox;  and  the  instant  it  pass- 
ed that  point,  ihe  long  night  of  the  north  pole  commences,  and 

49.  How  ia  the  north  pole  inclined  in  the  middle  of  our  summer,  and 
what  effect  has  this  on  the  north  frigid  zone?  50,  In  what  direction  does 
the  north  pole  always  point?  51.  What  is  shown  by  the  position  ,  he  earth 
at  B,  in  the  figure  ?  52.  How  does  the  sun  then  shine  at  the  poles,  and  what 
is  the  effect  on  the  days  and  nights  ? 


ON   THE    EARTH.  lOX 

the  south  pole  begins  to  enjoy  the  light  of  the  sun.  We  shall 
now  make  the  earth  proceed  in  its  orbit,  and  you  may  observe 
that  as  it  advances,  the  days  shorten  and  the  nights  lengthen, 
throughout  the  northern  hemisphere,  until  it  arrives  at  the  win- 
ter solstice,  on  the  21st  of  December,  when  the  north  frigid  zone 
is  entirely  in  darkness,  and  the  southern  has  uninterrupted  day- 
light. 

Caroline,  Then,  after  all,  the  sun  which  I  thought  so  partial, 
confers  his  favours  equally  on  all. 

Mrs.  B.  Not  so  either:  the  inhabitants  of  the  torrid  zone 
have  much  more  heat  than  we  have,  as  the  sun's  rays  fall  per- 
pendicularly twice  in  the  course  of  a  year,  on  every  place  within 
tlie  tropics,  while  they  shine  more  or  less  obliquely  on  the 
rest  of  the  world,  and  almost  horizontally  at  the  poles;  for  dur- 
ing their  long  day  of  six  months,  the  sun  moves  round  their  ho- 
rizon without  either  rising  or  setting;  the  only  observable  dif- 
erence,  is  that  it  is  more  elevated  by  a  few  degrees  at  mid-day, 
than  at  midnight. 

Emily.  To  a  person  placed  in  the  temperate  zone,  in  the 
situation  in  which  we  are  in  England,  the  sun  will  shine  neither 
so  obliquely  as  it  does  on  the  poles,  nor  vertically  as  at  the 
equator;  but  its  rays  will  fall  upon  him  more  obliquely  in  au- 
tumn, and  winter,  than  in  summer. 

Caroline.  And  therefore,  the  inhabitants  of  the  temperate 
zones,  will  not  have  merely  one  day,  and  one  night,  in  the  year, 
as  happens  at  the  poles,  nor  will  they  have  equal  days,  and  equal 
nights,  as  at  the  equator;  but  their  days  and  nights  will  vary  in 
length,  at  different  times  of  the  year,  according  as  their  respec- 
tive poles  incline  towards,  or  from  the  sun,  and  the  difference  will 
be  greater  in  proportion  to  their  distance  from  the  equator. 

Mrs.  B.  We  shall  now  follow  the  earth  through  the  other 
half  of  her  orbit,  and  you  will  observe,  that  now  exactly  the 
same  changes  take  place  in  the  southern  liemisphere,  as  tliose  we 
have  just  remarked  in  the  northern.  Day  commences  at  the 
south  pole,  when  night  sets  in  at  the  north  pole;  and  in  every 
other  part  of  the  southern  hemisphere  the  days  are  longer  than 
the  nights,  while,  on  the  contrary,  our  nights  are  longer  than 
our  days.  When  the  earth  arrives  at  the  vernal  equinox,  D, 
where  the  ecliptic  again  cuts  the  equator,  on  the  21st  of  March, 
she  is  situated,  with  respect  to  the  sun,  exactly  in  the  same 
position,  as  in  the  autumnal  equinox;  and  the  only  difference 

53.  When  the  earth  has  passed  the  autumnal  equinox,  what  changes  take 
place  at  the  poles,  and  also  in  the  whole  northern  and  southern  hemispheres  ? 
54.  Why  is  the  heat  greatest  within  the  torrid  zone  ?  55.  How  does  the  sua 
appear  at  the  poles,  during  the  period  of  day  there  ?  56.  In  what  will  the  days 
and  nights  differ  in  the  temperate  zone,  from  those  at  the  poles,  and  at  the 
equator  ? 

I  2 


102  ON   THE    EARTH. 

with  respect  io  the  earth,  is,  that  it  is  now  autumn  in  the 
southern  hemisphere,  whilst  it  is  Spring  with  us. 

Caroline.  Then  the  days  and  nights  are  again  every  where 
equal. 

Mrs.  B.  Yes,  for  the  half  of  the  globe  which  is  enlightened, 
extends  exactly  from  one  pole  to  the  other,  the  sun  has  just 
risen  to  the  north  pole,  and  is  just  setting  to  the  south  pole  5  but 
in  every  other  part  of  the  globe,  the  day  and  night  is  of  twelve 
hours  length;  hence  the  word  equinox,  which  is  derived  from 
the  Latin,  meaning  equal  night. 

As  our  summer  advances,  the  days  lengthen  in  the  northern 
hemisphere,  and  shorten  in  the  southern,  till  the  earth  reaches 
the  summer  solstice,  when  the  north  frigid  zone  is  entirely 
iliumined,  and  the  southern  is  in  complete  darkness;  and  we 
have  now  brought  the  earth  again  to  the  spot  from  whence  we 
first  accompanied  her. 

Emily.  This  is  indeed  a  most  satisfactory  explanation  of  the 
cause  of  the  diiferent  lengths  of  our  days  and  nights,  and  of 
the  variation  of  the  seasons;  and  the  more  I  learn,  the  more  I 
admire  the  simplicity  of  means  by  which  such  wonderful  eftects 
are  produced. 

Mrs.  B.  I  Cnow  not  which  is  most  worthy  of  our  admiration, 
the  causes,  or  the  effects  of  the  earth's  revolution  round  the  sun. 
The  mind  can  find  no  object  of  contemplation  more  sublime, 
than  the  course  of  this  magnificent  globe,  impelled  by  the  com- 
bined powers  of  projection  and  attraction,  to  roll  in  one  invaria- 
ble course,  around  the  source  of  light  and  heat:  and  what  can  be 
more  delightful  than  the  beneficent  effects  of  this  vivifjing 
power  on  its  attendant  planet.  It  is  at  once  the  gra.nd  pnnci- 
ple  which  animates  and  fecundates  nature. 

Emily.  There  is  one  circumstance  in  which  this  little  ivory 
globe  appeai-s  to  me  to  differ  from  the  earth;  it  is  not  quite  dark 
on  that  side  of  it  which  is  turned  from  the  candle,  as  is  the  case 
with  the  earth  when  neither  moon  nor  stars  are  visible. 

Mrs.  B.  This  is  owing  to  the  light  of  the  pandle,  being  re- 
jflected  by  the  walls  of  the  room,  on  every  part  of  the  globe,  con- 
sequently that  side  of  the  globe,  on  which  the  candle  does  not 
directly  shine,  is  not  in  total  darkness.  Now  the  skies  have  no 
walls  to  reflect  the  sun's  light  on  that  side  of  our  earth  which  is 
in  darkness. 

Caroline.  I  beg  vour  pardon,  Mrs.  B.,  I  think  that  the  moon, 
and  stars,  answer  the  purpose  of  walls  in  reflecting  the  sun's 
light  to  us  in  the  night. 

57.  Trace  the  earth  from  the  winter  solstice  to  the  vernal  equiuox,  and  in- 
form me  what  changes  take  place.  58.  What  takes  place  at  the  time  of  the 
yernal  equinox,  and  what  is  meant  by  the  term  ?  59.  In  proceeding  from  the 
rexTwd  equinox  to  the  summer  solstice,  what  changes  take  place  .^ 


m 


ON   THE    EARTH.  103 

Mrs.  B.  Very  well,  Caroline;  that  is  to  say,  the  moon  and 
planets;  for  the  fixed  stars,  you  know,  shine  by  their  own  light. 

Emily,  You  say,  that  the  superior  heat  of  the  equatorial 
parts  of  the  earth,  arises'  from  the  rays  falling  perpendicularly 
on  those  regions^  whilst  they  fall,  obliquely  on  these  more  north- 
ern regions;  now  I  do  not  understand  why  perpendicular  rays 
should  afford  more  heat  than  oblique  rays. 

Caroline,  You  need  only  hold  your  hand  pei*pendicularly 
over  the  candle,  and  then  hold  it  sideways  obliquely,  to  be  sen- 
sible of  the  difference. 

Emily.     I  do  not  doubt  the  fact,  but  I  wish  to  have  it  explained . 

Airs.  B.  You  are  quite  right;  if  Caroline  had  not  been  satis- 
fied with  ascertaining  the  fact,  without  understanding  it,  she 
would  not  have  brought  forward  the  candle  as  an  illustration; 
the  reason  why  you  feel  so  much  more  heat  if  you  hold  your 
hand  perpendicularly  over  the  candle,  than  if  you  liold  it  side- 
ways, is  because  a  stream  of  heated  vapour  constantly  ascends 
from  the  candle,  or  any  otlier  burning  body,  which  bein^  lighter 
than  the  air  of  the  room,  does  not  spread  laterally  but  rises  per,' 
pendicularly,  and  this  led  you  to  suppose  that  the  rays  were 
notter  in  tlie  latter  direction.  Had  you  reflected,  you  would 
liave  discovered  that  rays  issuing  from  the  candle  sideways,  are 
no  less  perpendicular  to  your  hand  when  held  opposite  to  them, 
than  the  rays  which  ascend  when  your  hand  is  held  over  them. 

The  reason  why  the  sun's  rays  afford  less  heat  when  in  an 
oblique  direction,  than  when  perpendicular,  is  because  fewer  of 
them  fall  upon  an  equal  portion  of  the  earth;  this  will  be  under- 
stood better  by  referring  to  plate  10.  fig.  1,  which  represents 
two  equal  portions  of  the  sun's  rays,  shining  upon  different  parts 
of  the  earth.  Here  it  is  evident,  that  the  same  quantity  of 
rays  fall  on  the  space  A  B,  as  fall  on  the  space  B  C;  and  as  A 
B  is  less  than  B  C,  the  heat  and  light  will  be  mucli  stronger  in 
the  former  than  in  the  latter;  A  B,  you  see,  represents  the 
equatorial  regions,  where  the  sun  shines  perpendicularly;  and  B 
C,  the  temperate  and  frozen  climates,  where  his  rays  fall  more 
obliquely.\ 

Emily.  This  accounts  not  only  for  the  greater  heat  of  the 
equatorial  regions,  buit  for  the  greater  heat  of  our  summers,  as 
^he  sun  shines  less  obliquely  in  summer  than  in  winter. 

Mrs,  B.  This  you  will  see  exemplified  in  figure  2,  in  which 
the  earth  is  represented,  as  it  is  situated  on  the  21st  of  June, 
and  England  receives  less  oblique,  and  consequently  a  greater 

60.  From  what  cause  arises  the  superior  heat  of  the  equatorialTegions  ? 
61.  Why  should  oblique  rays  afford  less  heat  than  those  which  are  perpendi- 
cular? 62.  How  is  this  explained  by  fig.  1.  plate  10?  63.  How  do  you  ac- 
count for  tlie  superior  heat  of  summer,  and  how  is  this  exemplified  in  fig.  2 
and  3,  plate  10  ? 


104  ON   THE    EARTH. 

number  of  rays,  than  at  any  other  season;  and  figure  3,  shows 
the  situation  of  England  on  the  21st  of  December,  when  the 
rays  of  the  sun  fall  most  obliquely  upon  her.)  But  there  is  also 
another  reason  why  oblique  rays  give  less  heat,  than  perpendi- 
cular rays;  which  is,  that  (they  have  a  greater  portion  of  the  at- 
mosphere to  traverse;  and  tkough  it  is  true,  that  the  atmosphere 
is  itself  a  transparent  body,  freely  admitting  the  passage  of  the 
sun's  rays,  yet  it  is  alw  ays  loaded  more  or  less  with  dense  and 
foggy  vapour,  which  the  rays  of  the  sun  cannot  easily  penetrate^ 
therefore,  the  greater  the  quantity  of  atmosphere  the  sun's  rays 
have  to  pass  through  in  their  way  to  the  eartn,  the  less  heat  they 
will  retain  when  they  reach  it.  This  will  be  better  understood, 
by  referring  to  fig.  4.  The  dotted  line  round  the  earth,  de- 
scribes the  extent  of  the  atmosphere,  and  the  lines  which  pro- 
ceed from  the  sun  to  the  earth,  the  passage  of  two  equal  por- 
tions of  the  sun's  rays,  to  the  equatorial  and  polar  regions;  the 
latter  you  see,  from  its  greater  obliquity,  passes  through  a  great- 
er extent  of  atmosphere. 

Caroline.  And  this,  no  doubt,  is  the  reason  why  the  sun,  in 
the  morning  and  in  tlie  evening,  gives  so  much  less  heat,  than 
at  mid -day. 

Mrs.  B.  The  diminution  of  heat,  morning  and  evening,  is 
certainly  owing  to  the  greater  obliquity  of  the  sun's  rays;  and 
they  are  also  affected  by  i\\^  other,  both  the  cause,  which  1  have 
just  explained  to  you;  the  difficulty  of  passing  through  a  foggy 
atmosphere  is  perlmps  more  particulai'ly  applicable  to  them,  as 
mist  and  vapours  are  prevalent  about  the  time  of  sunrise  and 
sunset.  But  the  diminished  obliquity  of  the  sun's  rays,  is  not 
the  sole  cause  of  the  heat  of  summer;  the  length  of  the  days 
greatly  conduces  to  it;  for  the  longer  the  sun  is  above  the  hori- 
zon, tlie  more  heat  he  will  communicate  to  the  earth. 

Caroline.  Both  the  longest  days,  and  the  most  perpendicular 
rays,  are  on  the  21st  of  June;  and  yet  the  greatest  heat  prevails 
in  July  and  August. 

Mrs.  B.  Those  parts  of  the  earth  which  are  once  heated, 
retain  the  heat  for  some  length  of  time,  and  the  additional  heat 
they  receive,  occasions  an  elevation  of  temperature,>  although 
the  days  begin  to  shorten,  and  the  sun's  rays  to  fall  more  ob- 
liquely. For  the  same  reason,  we  have  generally  more  heat  at 
three  o'clock  in  the  afternoon,  than  at  twelve,  when  the  sun  is 
on  the  meridian. 

64.  What  other  cause  lessens  the  intensity  of  oblique  rays  ?  65.  How  is 
this  explained  by  fig.  4.'*  66.  What  causes  conspire  to  lessen  the  solar  heat 
in  the  morning  and  evening  ?  67.  The  greatest  heat  of  summer  is  after  the 
solstice,  and  the  greatest  heat  of  the  day,  after  12  o'clock,  although  the  sun's 
rays  are  then  most  direct,  how  is  this  accounted  for  ^ 


ON   THE    EARTH.  105 

Emily.  And  pray,  have  the  other  planets  the  same  vicissi- 
tudes ot  seasons,  as  the  earth? 

Mrs,  B,  Some  of  them  more,  some  less,  according  as  their 
axes  deviate  more  or  less  from  the  perpendicuiar,  to  the  plane  of 
their  orbits.  The  axis  of  Jupiter,  is  nearly  perpendicular  to  the 
plane  of  his  orbit;  the  axes  of  Mars,  and  of  Saturn,  are  each,  in- 
clined at  angles  of  about  sixty  degrees;  whilst  the  axis  of  Venus 
is  believed  to  be  elevated  only  fifteen  or  twenty  degrees  above 
her  orbit;  the  vicissitudes  of  her  seasons  must  therefore  be  con- 
siderably greater  than  ours.  For  further  particulars  respecting 
the  planets,  I  shall  refer  you  to  Bonnycastle's  Introduction  to 
Astronomy. 

I  have  but  one  more  observation  to  make  to  you,  relative  to 
the  earth's  motion;  which  is,  that  although  we  have  but  365  days 
and  nights  in  the  year,  she  performs  366  complete  revolutions  on 
her  axis,  during  that  time. 

Caroline.  How  is  that  possible?  for  every  convplete  revolu- 
tion must  bring  the  same  place  back  to  the  sun.  It  is  now  just 
twelve  o'clock,  the  sun  is,  therefore,  on  our  meridian;  in  twen- 
ty-four hours  will  it  not  have  returned  to  our  meridian  again, 
and  will  not  the  earth  have-  made  a  complete  rotation  on  its 
axis? 

Mrs.  B.  If  the  earth  had  no  progressive  motion  in  its  orbit 
whilst  it  revolves  on  its  axis,  this  would  be  the  case;  but  as  it 
advances  almost  a  degree  westward  in  its  orbit,  in  the  same  time 
that  it  completes  a  revolution  eastward  on  its  axis,  it  must  re- 
volve nearly  one  degree  more  in  order  to  bring  the  same  meri- 
dian back  to  the  sun. 

Caroline.  Oh,  yes !  it  will  require  as  much  more  of  a  second 
revolution  to  bring  the  same  meridian  back  to  the  sun,  as  is- equal 
to  the  space  the  earth  has  advanced  in  her  orbit;  that  is,  nearly 
a  degree;  this  difference  is,  however,  very  little. 

Mrs.  B.  These  small  daily  portions  of  rotation,  are  each 
equal  to  the  three  hundred  and.  sixty -fifth  part  of  a  circle,  which 
at  the  end  of  the  year  amounts  to  one  complete  rotation. 

Emily.  That  is  extremely  curious.  If  the  earth  then,  had 
no  other  than  its  diurnal  motion,  we  should  have  366  days  in  the 
year. 

Mrs.  B.  We  should  have  366  days  in  the  same  period  of 
time  that  we  now  have  365;  but  if  we  did  not  revolve  round  the 
sun,  we  should  have  no  natural  means  of  computing  years. 

You  will  be  surprised  to  hear,  that  if  time  is  calculated  by  the 

68.  Is  there  any  change  of  seasons  in  the  other  planets  ?  69.  What  is  said 
respecting  the  axes  of  Jupiter,  of  Mars,  and  of  Saturn  ?  70.  In  365  days, 
how  many  times  does  the  earth  revolve  on  its  axis?  71.  How  is  this  ac- 
counted for  ?  72.  Do  the  fixed  stars  require  the  same  time  as  the  sun,  to  re- 
turn to  the  same  meridian .' 


106  ON   THE    EARTH. 

stars  instead  of  the  sun,  the  irregularity  which  we  have  just,no- 
ticed  does  not  occur,  and  that  one  complete  rotation  of  the  earth 
on  its  axis,  brings  the  same  meridian  back  to  any  fixed  star. 

Emily.  That  seems  quite  unaccountable;  for  the  earth  ad- 
vances in  her  orbit  with  regard  to  the  fixed  stars,  the  same  as 
with  regard  to  the  sun. 

Mrs,  B.  True,  but  then:%e  distance  of  the  fixed  stars  is  so 
immense,  that  our  solar  system  is  in  comparison  to  it  but  a  spot, 
and  the  whole  extent  of  the  earth's  orbit  but  a  pointy,  therefore, 
whether  the  earth  remain  stationary,  or  whether  it  revolved  in 
its  orbit  during  its  rotation  on  its  axis,  no  sensible  difference 
would  be  produced  with  regard  to  the  fixed  stars.  One  com- 
plete revolution  brings  the  same  meridian  back  to  the  same  fixed 
star;  hence  the  fixed  stars  appear  to  go  round  the  earth  in  a 
shorter  time  than  the  sun  by  three  minutes  fifty-six  seconds  of 
time. 

Caroline,  These  three  minutes  fifty-six  seconds  is  the  time 
which  the  earth  takes  to  perform  the  additional  three  hundred 
and  sixty-fifth  part  of  the  circle,  in  order  to  bring  the  same  me- 
ridian back  to  the  sun. 

Mrs.  B.  Precisely.  Hence  the  stars  gain  every  day  three 
minutes  fifty-six  seconds  on  the  sun,  which  makes  them  rise  that 
portion  of  time  earlier  every  day. 

When  time  is  calculated  by  the  stars  it  is  called  sidereal 
time;  when  by  the  sun,  solar,  or  apparent  time. 

Caroline.  Then  a  sidereal  day  is  three  minutes  fifty-six  se- 
conds shorter,  than  a  solar  day  oi  twenty -four  hours. 

Mrs.  B.  I  must  also  explain  to  you  what  is  meant  by  a  side- 
real year. 

The  common  year,  called  the  solar  or  tropical  year,  contain- 
ing 365  days,  five  hours,  forty-eight  minutes  and  fifty -two  se- 
conds, is  measured  from  the  time  the  sun  sets  out  from  one  of  the 
equinoxes,  or  solstices,  till  it  returns  to  the  same  again;  but  this 
year  is  completed,  before  the  earth  has  finished  one  entire  revolu- 
tion in  its  orbit. 

Emily.  I  thought  that  the  earth  performed  one  complete  re- 
volution in  its  orbit,  every  year;  what  is  the  reason  of  this  varia- 
tion? 

Mrs.  B.  It  is  owing  to  the  spheroidal  figure  of  the  earth. 
The  elevation  about  the  equator  produces  much  the  same  effect 
as  if  a  similar  mass  of  matter,  collected  in  the  form  of  a  moon, 
revolved  round  the  equator.  When  this  moon  acted  on  the 
earth,  in  conjunction  with,  or  in  opposition  to  the  sun,  variations 

73.  How  is  this  accounted  for  ?  74.  What  is  meant  by  the  solar  and  the 
sidereal  day?  75.  What  is  the  diflference  in  time  between  them  ?  To.  What 
is  the  length  of  the  tropical  year  '' 


"^L^ 

% 


'Hi^ 


Pl.iA.TEJg. 


I  ON   THE    EARTH.  107 

in  the  earth's  motion  would  be  occasioned,  and  these  variations 
produce  what  is  called  the  precession  of  the  equinoxes. 

Emily,  What  does  that  mean?  I  thought  the  equinoctial 
points,  were  fixed  points  in  the  heavens,  in  which  the  equator 
cuts  tlie  ecliptic. 

Mrs,  B,  These  points  are  not  quite  fixed,  but  have  an  appa- 
rently retrograde  motion,  among  the  signs  of  the  zodiac;  that  is 
to  say,  instead  of  being  at  every  revolution  in  the  same  place, 
they  move  backwards.  Thus  if  the.  vernal  equinox  is  at  A,  (fig. 
1.  plate  XI.)  the  autumnal  one,  will  be  at  B,  instead  of  C^and  the 
following  vernal  equinox,  at  D,  instead  of  at  A,  as  would  be  the 
case  if  tlie  equinoxes  were  stationary,  at  opposite  points  of  the 
earth's  orbit. 

Caroline.  So  that  when  the  earth  moves  from  one  equinox  to 
the  other,  though  it  takes  half  a  year  to  perform  the  journey,  it 
has  not  travelled  through  half  its  orbit. 

Mrs.  B.  And,  consequently,  when  it  returns  again  to  tlie 
first  equinox,  it  has  not  completed  the  whole  of  its  orbit.  In 
order  to  ascertain  when  the  earth  has  performed  an  entire  revo- 
lution in  its  orbit,  we  must  observe  when  the  sun  returns  in  con- 
junction witli  any  fixed  star;  and  this  is  called  a  sidereal  year. 
Supposing  a  fixed  star  situated  at  E,  (fig.  1.  plate  XI.)  the  sun 
would  not  appear  in  conjunction  with  it,  till  the  earth  had  return- 
ed to  A,  when  it  would  have  completed  its  orbit. 

Emily.  And  how  much  longer  is  the  sidereal,  than  the  solar 
year? 

Mrs.  B.  Only  twenty  minutes;  so  that  the  variation  of  the 
equinoctial  points  is  very  inconsiderable.  I  have  given  them  a 
greater  extent  in  the  figure,  in  order  to  render  them  sensible. 

In  regard  to  time,  I  must  further  add,  that^the  earth's  diurnal 
motion  on  an  inclined  axis,  together  with  its  annual  revolution 
in  an  elliptic  orbit,  occasions  so  much  complication  in  its  motion, 
as  to  produce  many  irregularities)  therefore  the  true  time  can- 
not be  measured  by  the  apparent  place  of  the  sun.  A  peifectly 
correct  clock,  would  in  some  parts  of  the  year  be  before  the  sun, 
and  in  other  parts  after  it.  There  are  but  four  periods  in  which 
the  sun  and  a  perfect  clock  would  agree,  which  is  the  15th  of 
April,  the  l6tJi  of  June,  the  2Sd  of  August,  and  the  24th  of  De- 
cember. 

77.  The  solar  year  is  completed  before  the  earth  has  made  a  complete  revo- 
lution in  its  orbit,  by  what  is  this  caused  ?  78.  What  is  this  called,  and  what 
is  represented  respecting  it  by  fig.  1,  plate  11  ?  79.  By  what  means  can  we 
ascertain  the  period  of  a  complete  revolution  of  the  earth  in  its  orbit,  as  illus- 
trated by  the  fixed  star  E,  in  fig.  1  ?  80.  What  difference  is  there  in  the  leng:th 
of  the  solar  and  sidereal  year?  81.  Why  can  we  not  always -ascertain  the 
true  time  by  the  apparent  place  of  the  sun  ? 


108  ON   THE    MOON. 

Emily.  And  is  there  any  considerable  difference  between 
solar  time,  and  true  time? 

Mrs.  B.  The  greatest  difference  amounts  to  between  fifteen 
and  sixteen  minutes.  Tables  of  equation  are  constructed  for  the 
purpose  of  pointing  out,  and  correcting  these  differences  between 
solar  time  and  equal  or  mean  time,  which  is  the  denomination 
given  by  astronomers,  to  true  time. 

82.  What  would  be  the  greatest  difference  between  solar,  and  true  time, 
as  indicated  by  a  perfect  clock  ? 


CONVERSATION  IX, 


ON  THE  MOON. 

•F  THE  moon's  motion. — PHASES  OF  THE  MOON. — ECLIPSES  OF  THE 
MOON. — ECLIPSES  OF  JUPITER's  MOONS. — OF  LATITUDE  AND  LONGI- 
TUDE.— OF  THE  TRANSITS  OF  THE  INFERIOR  PLANETS. — OF  THE  TIDES. 

MRS.    B. 

We  shall,  to-day,  confine  our  attention  to  the  moon,  which 
offers  many  interesting  phenomena. 

The  moon  revolves  round  the  earth  in  the  space  of  about 
twenty -nine  days  and  a  half;  in  an  orbit,  the  plane  of  which  is 
inclined  upwards  of  five  degrees  to  that  of  the  earth;  she  ac- 
companies us  in  our  revolution  round  the  sun." 

Emily.  Her  motion  then  must  be  of  a  complicated  nature; 
for  as  the  earth  is  not  stationary,  but  advances  in  her  orbit, 
whilst  the  moon  goes  round  her,  the  moon,  in  passing  round 
the  sun,  must  proceed  in  a  sort  of  scolloped  circle. 

Mrs.  B.  That  is  true;  and  there  are  also  other  circumstances 
which  interfere  with  the  simplicity,  and  regularity  of  the  moon's 
motion,  but  which  are  too  intricate  for  you  to  understand  at 
present. 

The  moon  always  presents  the  same  face  to  us,  by  which  it 
is  evident  that  she  turns  but  once  upon  her  axis,  while  she  per- 

1.  In  what  time  does  the  moon  revolve  round  the  earth?  what  is  the  incli- 
oation  of  her  orbit .''  and  how  does  she  accompany  the  earth  i*  2.  As  the 
moon  revolves  round  the  earth,  and  also  accompanies  it  in  its  annual  revo- 
lution, in  what  form  would  you  draw  the  moon's  orbit  ? 


ON    THE    MOON.  109 

forms  a  revolution  round  the  earth;  so  that  the  inhabitants  of 
the  moon  have  but  one  day,  and  one  night,  in  the  course  of  a 
lunar  month. 

Caroline,  We  afforrl  them,  however,  the  advantage  of  a 
magnificent  moon  to  enligliten  their  lon^  nights. 

Mrs.  B,  That  advantage  is  put  partial;  for  since  we  always 
see  the  same  hemisphere  of  tlie  moon,  the  inhabitants  of  that 
hemisphere  alone,  can  perceive  us. 

Caroline.  One  half  of  the  moon  then  enjoys  our  light,  while 
the  other  half  has  constantly  nights  of  darkness.  If  there  are 
any  astronomers  in  those  regions,  they  would  doubtless  be' 
tempted  to  visit  the  other  hemisphere,  in  order  to  behold  so 
grand  a  luminary  as  we  must  appear  to  them.  But,  pray,  do 
they  see  the  earth  under  all  the  changes,  which  the  moon  exhi- 
bits to  us.*^ 

Mrs.B.  Exactly  so.  These  changes  are  called^ the  phases 
of  the  moon,  and  require  some  explanation.  In  fig.  2,  plate  11, 
let  us  say,  that  S  represents  the  sun,  E  the  earth,  and  A  B  C  D 
E  F  G  H,  the  moon,  in  different  parts  of  her  orbit.  When  the 
moon  is  at  A,  her  dark  side  being  turned  towards  the  earth,  we 
shall  not  see  her  as  at  «;  but  her  disappearance  is  of  very  short, 
duration,  and  as  she  advances  in  her  orbit,  we  perceive  her  un- 
der the  form  of  a  new  moon:  when  she  has  gone  throu£;h  one 
eighth  of  her  orbit  at  B,  one  quarter  of  her  enlightened  nemis- 
phere  will  be  turned  towards  the  earth,  and  she  will  then  appear 
horned  as  at  h;  when  she  has  performed  one  quarter  of  her  orbit, 
she  shows  us  one  half  of  her  enlightened  side,  as  at  c,  and  this  is 
called  her  first  quarter;  at  d  she  is  said  to  be  gibbous,  and  at  e 
the  whole  of  the  enlightened  side  appears  to  us,  and  the  moon 
is  at  full.  As  she  proceeds  in  her  orbit,  she  becomes  again  gib- 
bous, and  her  enlightened  hemisphere  turns  gradually  away  from 
us,  until  vshe  arrives  at  G,  which  is  her  third  quarter;  proceeding 
thence  she  completes  her  orbit  and  disappears,  and  then  again 
resumes  her  form  of  a  new  moon,  and  passes  successively, 
through  the  same  changes. 

When  the  moon  is  new,  she  is  said  to  be  in  conjunction  with 
the  sun,  as  they  are  then  both  in  the  same  direction  from  the 
earth;  at  the  time  of  full  moon,  she  is  said  to  be  in  opposition, 
because  she  and  the  sun,  are  at  opposite  sides  of  the  eartn;  at  the 
time  of  her  first  and  third  quarters,  she  is  said  to  be  in  her  quad- 

3.  What  causes  the  moon  always  to  present  the  same  face  to  the  earth, 
and  what  must  be  the  len^h  of  a  day  and  night  to  its  inhabitants  ?  4.  Can 
the  earth  be  seen  from  every  part  of  Uie  moon,  and  will  it  always  exhibit  the 
same  appearance  ?  5.  What  are  the  changes  of  the  moon  called  ?  6.  How 
are  these  changes  explained  by  fig.  2.  plate  11?  7.  What  is  meant  by  her 
first  quarter?  8.  What  by  her  being  horned,  and  her  being  gibbous? 
9.  What  J)y  her  being  full?     10.  What  by  her  third  quarter  ? 


110  ON    THE    MOON. 

ratures,  because  she  is  then  one-fourth  of  a  circle,  or  90°,  from 
her  conjunction,  or  the  period  of  new  moon. 

Umiiy.  Are  not  the  eclipses  of  the  sun  produced  bj  the  moon 
passing  between  the  sun  and  the  earth  ? 

Mrs.  B.  Yes;  when  the  moon  passes  between  the  sun  and 
the  earth,  she  intercepts  his  rajs,  oi',  in  other  words,  casts  a 
shadow  on  the  earth,  then  tlie  sun  is  eclipsed,  and  daylight 
gives  place  to  darkness,  while  the  moon's  shadow  is  passing 
over  us. 

When,  on  the  contrary,  the  earth  is  between  the  sun  and  the 
moon,  it  is  we  who  intercept  the  sun's  rays,  and  cast  a  shadow 
im  the  moon;  she  is  then  said  to  be  eclipsed,  and  disappears 
from  our  view. 

Emily,  But  as  the  moon  goes  round  the  earth  every  month, 
she  must  be,  once  during  that  time,  between  the  earth  and  the 
sun;  and  the  earth  must  likewise  be  once  between  the  sun  and 
the  moon,  and  yet  we  have  not  a  solar  and  a  lunar  eclipse  every 
month?  • 

Mrs,  B.  I  have  already  informed  you,  that  the  orbits  of  the 
earth  and  moon  are  not  in  the  same  plane,  but  cross  or  intersect 
each  other;  and  the  moon  generally  passes  either  above  or  below 
that  of  the  earth,  when  she  is  in  conjunction  with  the  sun,  and 
does  not  therefore  intercept  its  rays,  and  produce  an  eclipse; 
for  this  can  take  place  only  when  the  moon  is  in,  or  near  her 
nodes,  which  is  the  name  given  to'those  two  points  in  which  her 
orbit  crosses  that  of  the  earth?-  eclipses  cannot  happen  at  any 
other  time,  because  it  i;!  then  only,  that  they  are  both  in  a  right 
line  with  the  sun. 

Emily.  And  a  partial  eclipse  of  the  moon  takes  place,  I  sup- 
pose, when,  in  passing  by  the  earth,  she  is  not  sufficiently  above 
or  below  the  shadow,  to  escape  it  entirely? 

Mrs.  B.  Yes,  one  edge  oi  her  disk  then  dips  into  the  shadow, 
and  is  eclipsed;  but  as  the  earth  is  larger  than  the  moon,  when 
eclipses  happen  precisely  at  the  nodes,  they  are  not  only  total, 
but  last  for  upwards  of  three  hours. 

A  total  eclipse  of  the  sun  rarely  occurs,  and  when  it  happens, 
the  total  darkness  is  confined  to  one  particular  part  of  the  earth, 
the  diameter  of  the  shadow  not  exceeding  180  miles;  evidently 
showing  that  the  moon  is  smaller  tlian  the  sun,  since  she  cannot; 

11.  What  is  meant  by  her  conjunction  ? — what  by  her  being  in  opposition  ? 
— ^what  by  her  quadratures?  12.  By  what  are  eclipses  of  the  sun  caused? 
13.  What  causes  eclipses  of  the  moon  ?  14.  What  is  meant  by  the  moon's 
nodes?  15.  Why  do  not  eclipses  happen  at  every  new  and  full  moon? 
16.  What  causes  partial  eclipses  of  the  moon?  17.  When  the  moon  is  ex- 
actly in  one  of  her  nodes,  what  length  of  time  will  she  be  eclipsed?  18.  Are 
total  eclipses  of  the  sun  frequent,  and  when  tliey  happen  what  is  their 
extent. ■* 


PUATKXll. 


I     ^ 


C  ^  ►'  '^^^^. 


^     ON    THE    MOON.  Ill 

entirely  hide  it  from  the  earth.  In  fig.  1,  plate  12,  you  will 
find  a  solar  eclipse  described;  S  is  the  sun,  M  the  moon,  and  E 
the  earth;  and  the  moon's  shadow,  you  see,  is  not  lar;»e  enough 
to  cover  the  earth.^  The  lunar  eclipses,  on  the  contrary,  are 
visible  from  every  part  of  the  earth,  where  the  moon  is  above 
the  horizon;  and  we  discover,  by  the  length  of  time  wliich  the 
moon  is  pas^in^  through  the  earth's  shatlow,  that  it  would  be 
sufficient  to  eclipse  her  totally,  were  she  many  times  her  actual 
sizej^it  follows,  therefore,  that  the  earth  is  much  larger  than  the 
moon. 

.In  fig.  2,  S  represents  the  sun,  which  pours  forth  rays  of  light 
in  straight  lines,  in  everi^direction.  E  is  the  earth,  and  IM  the 
moon.  Now  a  ray  of  light  coming  from  one  extremity  of  the 
sun's  disk,  in  the  direction  A  B,  will  meet  another,  coming  from 
the  opposite  extremity,  in  the  direction  C  B;  the  shadow  of  the 
earth  cannot  therefore  extend  beyond  B;  as  the  sun  is  larger 
than  the  earth,  the  shadow  of  the  latter  is  conical,  or  in  the 
figure  of  a  suaar  loaf,  it  gradually  diminishes,  and  is  much 
smaller  than  the  earth  where  the  moon  passes  through  it,  and 
yet  we  find  the  moon  to  be,  not  only  totally  eclipsed,  but  to 
remain  for  a  considerable  length  of  time  in  darkness,  and  hence 
we  are  enabled  to  ascertain  its  real  dimensions. 

Emily.  When  the  moon  eclipses  the  sun  to  us,  we  must  be 
eclipsed  to  the  moon.^ 

Mrs.  B.  Certainly;  for  if  the  moon  intercepts  tlie  sun's 
ra]  s,  and  casts  a  shadow  on  us,  we  must  necessarily  disappeai 
to  the  moon,  but  only  partially,  as  in  fig.  1. 

Caroline.  There  must  be  a  great  number  of  eclipses  in  tb« 
distant  planets,  which  liave  so  many  moons.^ 

Mrs,  B.  Yes,  few  days  pass  without  an  eclipse  taking  place; 
for  among  the  number  of  satellites,  one  or  the  other  of  them  are 
continually  passing  either  between  their  primary  and  the  sun; 
or  between  the  planet,  and  each  othcci  Astronomers  are  so  well 
acquainted  with  the  motion  of  the  planets,  and  their  satellites, 
that  they  have  calculated  not  only  tne  eclipses  of  our  moon,  but 
those  of  Jupiter,  with  such  perfect  accuracy,  that  it  has  aftbrded 
a  means  of  ascertaining  the  longitude. 

Caroline.  But  is  it  not  very  easy  to  find  both  the  latitude 
and  longitude  of  any  place  by  a  map  or  globe? 

Mrs.  B.  If  you  know  where  you  are  situated,  there  is  no 
difficulty  in  ascertaining  the  latitude  or  longitude  of  the  place, 
by  referring  to  a  map;  but  supposing  that  you  had  been  a  length 

19.  What  does  this  prove  respecting  the  size  of  the  moon?  20.  What  is 
shown  in  lig.  1,  plate  12?  21.  How  are  lunar  eclipses  visible,  and  what  is 
proved   by  their  duration?     22.  What  is  illustrated   by  fig.  2,  plate  12? 

23.  What  remark  is  made  respecting  those  planets  which  have  several  moons  ? 

24.  What  use  is  made  of  the  eclipses  of  the  satellites  of  Jupiter.'* 


lis 


ON    THE    MOON. 


of  time  at  sea,  interrupted  in  your  course  by  storms,  a  map 
would  aftbrd  you  very  little  assistance  in  discovering  where  you 
were. 

Caroline.  Under  such  circumstances,  I  confess  I  should  be 
equally  at  a  loss  to  discover  either  latitude,  or  longitude. 

Mrs.  B.  The  latitude  is  usually  found  by  taking  the  alti- 
tude of  the  sun  at  mid -day;  that  is  to  say,  the  number  of  degrees 
that  it  is  elevated  above  the  horizon,  for  the  sun  appears  more 
elevated  as  we  approach  the  equator,  and  less  as  we  recede 
ft'om  it. 

Caroline.  But  unless  you  can  see  the  sun,  how  can  you  take 
its  altitude.-^  ^ 

Mrs.  B.  When  it  is  to^o  cloudy  to  see  the  sun,  the  latitude  is 
sometimes  found  at  nigh  t,i^by  the  polar  star;  the  north  pole  of  the 
eartli,  points  constantly  towards  one  particular  part  of  the  hea- 
vens, in  which  a  star  is  situated,  called  the  Polar  star:  this  star 
is  visible  on  clear  niojits,  from  every  part  of  the  northern  hemis- 
phere; the  altitude  of  the  polar  star,  is  therefore  the  same  number 
of  degrees,  as  that  of  the  pole;  the  latitude  may  also  be  deter- 
mined by  observations  made  on  any  of  the  fixed  stars  if-  the  situa- 
tion therefore  of  a  vessel  at  sea,  with  regard  to  nortli'and  south, 
is  easily  ascertained.  The  difficulty  is,  respecting  east  and  west, 
that  is  to  say,  its  longitude.  As  we  have  no  eastern  poles  from 
which  we  can  reckon  our  distance,  some  particular  spot,  or  line, 
must  be  fixed  upon  for  that  purpose.  The  ^English,  reckon  from 
the  meridian  of  Greenwich,  where  the  royal  observatory  is  situ- 
ated; in  French  maps,  you  will  find  that  the  longitude  is  reckon- 
ed from  the  meridian  of  Paris. 

The  rotation  of  the  earth  on  its  axis  in  24  hours  from  west  to 
east,  occasions,  you  know,  an  apparent  motion  of  the  sun  and 
stars  in  a  contrary  direction,  and  the  sun  appears  to  go  round 
the  earth  in  the  space  of  24  hours,  passing  over  fifteen  degrees, 
or  a  twenty -fourth  part  of  the  earth's  circumference  every  hour; 
therefore,  when  it  is  twelve  o'clock  in  London,  it  is  one  o'clock 
in  any  place  situated  fifteen  degrees  to  the  east  of  London,  as 
the  sun  must  have  passed  the  meridian  of  that  place,  an  hour  be- 
fore he  reaches  that  of  London.  For  the  same  reason  it  is 
eleven  o'clock  in  any  place  situated  fifteen  degrees  to  the  west 
of  London,  as  the  sun  will  not  come  to  that  meridian  till  an  hour 
later. 

If  then  the  captain  of  a  vessel  at  sea,  could  know  precisely 
what  was  the  hour  at  London,  he  could,  by  looking  at  his  watch, 

25.  How  is  the  latitude  of  a  place  usually  found  ?  26.  By  what  other 
means  may  latitude  be  found ?  27.  From  what  is  longitude  reckoned?  28. 
How  does  the  rotation  of  the  earth  upon  its  axis,  govern  the  time  at  different 
places  ? 


ON   THE    MOON.  117 

habitants  of  both  those  situations,  at  the  same  time.  Besides,  as 
the  orbit  of  the  moon  is  very  nearly  parallel  to  that  of  the  earth, 
she  is  never  vertical,  but  to  the  inhabitants  of  the  torrid  zone. 
.  Caroline.  In  the  torrid  zone,  then,  I  hope  you  will  giant  that 
the  moon  is  immediately  over,  or  opposite  the  spots  where  it  is 
high  water? 

Mrs.  B.  I  cannot  even  admit  that^  for  the  ocean  naturally 
partaking  of  the  earth's  motion,  in  its  rotation  from  west  to  east, 
the  moon,  in  forming  a  tide,  has  to  contend  against  the  eastern 
motion  of  the  waves.  AH  matter,  you  know,  by  its  inertia, 
makes  some  resistance  to  a  change  of  state 5  the  waters,  there- 
fore, do  not  readily  yield  to  the  attraction  of  the  moon,  and  the 
eftect  of  her  influence  is  not  complete,  till  three  hours  after  she 
has  passed  the  meridian,  where  it  is  full  tide. 

When  a  body  is  impelled  by  any  force,  its  motion  may  con- 
tinue, after  the  impelling  force  ceases  to  act:  this  is  the  case 
with  all  projectiles.  A  stone  thrown  from  the  hand,  continues 
its  motion  for  a  length  of  time,  proportioned  to  the  force  given  to 
it:  there  is  a  perfect  analogy  between  this  effect,  and  me  con- 
tinued rise  of  the  water,  after  the  moon  has  passed  the  meridian 
at  any  particular  place. 

Emily.  Pray  what  is  the  reason  that  the  tide  is  three-quar- 
ters of  an  hour  later  every  day.^ 

Mrs.  B.  Because  it  is  twenty-four  hours  and  three-quarters 
before  the  same  meridian,  on  our  globe,  returns  beneath  the  moon. 
The  earth  revolves  on  its  axis  in  about  twenty -four  hours;  if  the 
moon  were  stationary,  therefore,  the  same  part  of  our  globe 
would,  every  twenty-four  hours,  return  beneath  the  moon;  but  as 
during  our  daily  revolution,  the  moon  advances  in  her  orbit,  the 
earth  must  make  more  than  a  complete  rotation,  in  order  to  bring 
the  same  meridian  opposite  the  moon:  we  are  three-quarters  of 
an  hour  in  overtaking  her.  The  tides,  therefore,  are  retarded, 
for  the  same  reason  that  the  moon  rises  later  by  three-quarters  of 
an  hour,  every  day. 

We  have  now,  I  think,  concluded  the  observations  I  had  to 
make  to  you  on  the  subject  of  astronomy;  at  our  next  interview, 
I  shall  attempt  to  explain  to  you  the  elements  of  hydrostatics. 

45.  Why  in  the  open  ocean,  is  it  high  water,  some  hours  after  the  moon 
has  passed  the  meridian?  46.  Why  are  the  tides  three-quarters  of  an  hour 
later  every  day  ? 


CONVERSATION  X. 


ON  THE  MECHANICAL  PROPERTIES  OF  FLUIDS. 

DEFINITION  OF  A  FLUID. — DISTINCTION  BETWEEN  FLUIDS  AND  LlftUIDS. 
— OP  NON-ELASTIC  FLUIDS. — SCARCELY  SUSCEPTIBLE  OF  COMPRESSION. 
— OF  THE  COHESION  OP  FLUIDS. — OF  THEIR  GRAVITATION. — OF  THEIR 

EaUILIBRIUM. — OF     THEIR    PRESSURE. OF     SPECIFIC     GRAVITY. OF 

THE  SPECIFIC  GRAVITY  OF  BODIES  HEAVIER  THAN  WATER. OF  THOSE 

OF   THE    SAME   WEIGHT   AS    WATER. — OF   THOSE    LIGHTER   THAN   WA- 
TER.—OF  THE  SPECIFIC  GRAVITY  OF  FLUIDS. 

MRS.    B. 

We  have  hitherto  confined  our.  attention  to  the  mechanical 
properties  of  solid  bodies,  which  have  been  illustrated,  and,  I 
nope,  thoroughly  impressed  upon  your  memory,  by  the  conver- 
sations we  have  subsequently  had,  on  astronomy.  It  will  now  be 
necessary  for  me  to  give  you  some  account  of  the  mechanical 
properties  of  fluids— a  science  which,  when  applied  to  liquids,  is 
divided  into  two  parts,  hydrostatics  and  hydraulics.  Hydro- 
statics, treats  of  the  weight  and  pressure  of  fluids;  and  4iydrau- 
lics,  of  the  motion  of  fluids,  and  the  effects  produced  by  this 
motion.  A  fluid  isr  a  substance  which  yields  to  the  slightest 
pressure.  If  you  dip  your  hand  into  a  basin  of  water,  you  are 
scarcely  sensible  of  meeting  with  any  resistance. 

Emily.  The  attraction  of  cohesion  is  then,  I  suppose,  less 
powerful  in  fluids,  than  in  solids? 

Mrs.  B.  Yes;  fluids,  generally  speaking,  are  bodies  of  less 
density  than  solids.  From  tlie  sliglit  cohesion,  of  the  particles 
of  fluids,  and  the  facility  with  which  they  slide  over  each  other, 
it  is  inferred,  that  they  have  but  a  slight  attraction  for  each 
other,  and  that  this  attraction  is  equal,  in  every  position  of  their 

§  articles,  and  therefore  produces  no  resistance  to  a  perfect  free- 
om  of  motion  among  themselves. 
Caroline.     Pray  what  is  the  distinction  between  a  fluid  and 
a  liquid  ?  , 

Mrs.  B.     Liquids  comprehend  only  one  class  of  fluids.  There 

.  What  ai-e  the  two  divisions  of  the  science  which  treats  of  the  mechanical 
properties  of  liquids  ?     2.  Of  what  do  hydrostatics  and  hydraulics  treat  ?     Ll. 
What  is  a  fluid  defined  to  he  ?     4.  From  what  is  fluidity  supposed  to  ar! 
5,  Into  what  two  classes  are  fluids  divided  ? 


MECHANICAL    PROPERTIES    OF    FLUIDS.  119 

is  another  class,  distinguished  bj  the  name  of  elastic  fluids, 
or  gases,  which  compreliends  the  air  of  the  atmosphere,  and 
all  the  various  kinds  of  air  with  which  you  will  become  ac- 
quainted, when  you  study  chemistry.  Their  mechanical  pro- 
perties we  shall  examine  hereafter,  and  confine  our  attention 
this  morning,  to  those  of  liquids,  or  non-elastic  fluids. 

Water,  and  liquids  in  general,  are  scarcely  susceptible  of 
being  compressed,  or  squeezed  into  a  snialler  space,  than  that 
which  they  naturally  occupy.  Such,  however,  is  the  extreme 
minuteness  of  their  particles,  that  by  strong  compression,  they 
sometimes  force  their  way  through  the  pores  of  the  substance 
which  confines  them.  This  was  shown  by  a  celebrated  experi- 
ment, made  at  Florence  many  years  ago.  A  hollow  globe  of 
gold  was  filled  with  water,  and  on  its  being  submitted  to  great 
pressure,  the  water  was  seen  to  exude  through  the  pores  of  the 
gold,  which  it  covered  with  a  tine  dew.  Many  pnilosophers, 
however,  think  that  this  experiment  is  too  much  relied  upon,  as 
it  does  not  appear  that  it  has  ever  been  repeated 5  it  is  possible, 
therefore,  that  there  may  have  been  some  source  of  error,  which 
was  not  discovered  by  the  experimenters.  Fluids,  appear  to 
gravitate  more  freely,  than  solia  bodies;  for  the  strong  cohesive 
attraction  of  the  particles  of  the  latter,  in  some  measure  coun- 
teracts the  eff*ect  of  gravity.  In  this  table,  for  instance,  the 
cohesion  of  the  particles  of  wood,  enables  four  slender  legs  to 
support  a  considerable  weight.  Were  the  cohesion  destroyed, 
or,  m  other  words,  the  wood  converted  into  a  fluid,  no  suonort 
could  be  afforded  by  the  legs,  for  the  particles  no  longer  cofier- 
ing  together,  each  would  press  separately  and  independent! v, 
and  would  be  brought  to  a  level  with  the  surface  of  the  earth.  "^ 

Einily,  This  want  of  cohesion  is  then  tlie  reason  why  fluids 
can  never  be  formed  into  figures,  or  maintained  in  heaps;  for 
though  it  is  true  the  wind  raises  water  into  waves,  they  are  im- 
mediately afterwards  destroyed  by  gravity,  and  water  always 
finds  its  level. 

J\lrs.  B.  Do  you  understand  what  is  meant  by  the  level,  or 
equilibrium  of  fluids? 

Emily.     I  believe  I  do,  though  I  feel  rather  at  a  loss  to  ex 
plain  it.     Is  not  a  fluid  level  when  its  surface  is  smooth  and 
flat,  as  is  the  case  with  all  fluids,  when  in  a  state  of  rest? 

Mrs,  B.  Smooth,  if  you  please,  but  not  flat;  for  the  defini- 
tion of  the  equilibrium  of  a  fluid  is,  that  every  part  of  the  sur- 
face is  equally  distant  from  tlie  point  to  which  they  gravitate, 
that  is  to  say,  from  the  centre  of  the  earth;  hence  the  surface 

6.  What  is  said  of  the  incompressibility  of  liquids,  jand  what  experiment  is 
related  ?  7.  Ought  this  experiment  to  be  considered  as  conclusiye  ?  8.  Whj 
do  fluids  appear  to  gravitate  more  freely  than  solids  ? 


120  MECHANICAL    PROPERTIES    OF    FLUIDS. 

of  all  fluids  must  be  spherical,  not  flat,  since  ihej  will  partake 
of  the  spherical  form  of  the  globe.  This  is  very  evident  m  large 
bodies  of  water,  such  as  the  ocean,  but  the  sphericity  of  small 
bodies  of  water,  is  so  trifling,  that  their  surfaces  appear  flat. 

This  level,  or  equilibrium  of  fluids,  is  the  natural  result  of 
their  particles  gravitating  independently  of  each  others  for  when 
any  particle  of  a  fluid,  accidentally  finds  itself  elevated  above 
the  rest,  it  is  attracted  down  to  the  level  of  tlie  surface  of  the 
fluid,  and  the  readiness  with  which  fluids  yield  to  the  slightest 
impression,  will  enable  the  particle  by  its  weight,  to  penetrate 
the  surface  of  the  fluid,  and  mix  with  it. 

Caroline.  But  I  have  seen  a  drop  of  oil,  float  on  the  surface 
©f  water,  without  mixing  v/ith  it. 

Mrs.  B.  They  do  not  mix,  because  their  particles  repel  each 
©ther,  and  the  oil  rises  to  the  surface,  because  oil  is  a  lighter 
liquid  than  water.  If  you  were  to  pour  water  over  it,  the  oil 
would  still  rise,  being  forced  up  by  the  superior  gravity  of  the  ] 
water.  Here  is  an  instrument  called.^  spirit-level,  (fig.  1 ,  plate 
13.)  which  is  constructed  upon  the  principle  of  the  equilibrium 
of  fluids.  It  consists  of  a  snort  tube  A  B,  closed  at  both  ends, 
and  containing  a  little  water,  or  more  commonly  some  spirits: 
it  is  so  nearly  filled,  as  to  leave  only  a  small  bubble  of  air;  when 
tlie  tube  is  perfectly  horizontal,  this  bubble  will  occupy  the 
middle  of  it,  but  when  not  perfectly  horizontal,  the  water  runs 
to  the  lower,  and  the  bubble  of  air  or  spirit  rises  to  the  upper 
end;  by  this  instrument,  tlie  level  of  any  situation,  to  which  we 
apply  it,  may  be  ascertained. 

From  the  strong  cohesion  of  their  particles,  you  may  there- 
fore consider  solid  bodies  as  gravitating  in  masses,  while  every 
particle  of  a  fluid  may  be  considered  as  separate,  and  gravi- 
tating independently  of  each  other.  Hence  the  resistance  of 
a  fluid,  is  considerably  less,  than  that  of  a  solid  body;  for  the 
resistance  of  the  particles,  acting  separately,  is  more  easily 
overcome. 

Emily.  A  body  of  water,  in  falling,  does  certainly  less  injurj- 
than  a  solid  body  of  the  same  weight. 

Mrs.  B.  The  particles  of  fluids,  acting  thus  independent- 
ly, press  against  each  other  in  every  direction,  not  only  down- 
wards, but  upwards,  and  laterally  or  sideways;  and  in  conse- 
quence of  this  equality  of  pressure,  every  particle  remains  at 
rest,  in  the  fluid.     If  you  agitate  the  fluid,  you  disturb  this 

9.  "When  is  a  fluid  said  to  be  in  equilibrium  ?  10.  What  is  there  in  the 
nature  of  a  fluid,  which  causes  it  to  seek  this  level  ?  11.  What  circumstance! 
occasion  oil  to  float  upon  water  ?  12.  What  is  the  nature  and  use  of  the  in- 
strument represented  in  fig.  1,  plate  13  ?  13.  What  difference  is  there  in  the 
gnr&ritatiou  of  aolid  maseee,  and  of  fluids  i 


Jt*UATEiin. 


MECHANICAL    PROPERTIES    OF    FLUIDS.  121 

equality  of. pressure,  and  the  fluid  will  not  rest,  till  its  equili- 
brium IS  restored. 

Caroline.  Tl\e  pressure  downwards  is  very  natural;  it  is  the 
effect  of  gravity;  one  particle,  weighing  upon  another,  presses  on 
it;  but  the  pressure  sideways,  and  particularly  the  pressure  up- 
wards, I  cannot  understand. 

Mrs,  B.  If  there  were  no  lateral  pressure,  water  would  not 
run  out  of  an  opening  on  the  side  of  a  vessel.  If  you  fill  a  vessel 
with  sand,  it  will  not  continue  to  run  out  of  sucli  an  opening,  be- 
cause there  is  scarcely  any  lateral  pressure  among  its  particles. 

Emily.  When  water  runs  out  of  the  side  of  a  vessel,  is  it 
not  owing  to  the  weight  of  the  water,  above  the  opening? 

Mrs.  B.  If  the  particles  of  fluids  were  arranged  in  regular 
columns,  thus,  (fi|^.  2.)  there  would  be  no  lateral  pressure,  for 
when  one  particle  is  perpendicularly  above  the  other,  it  can  only 
press  downwards;  but  as  it  must  continually  happen,  that  a  par- 
ticle presses  between  two  particles  beneath,  (fig.  3.)  these  last, 
must  suffer  a  lateral  pressure. 

Emily.  The  same  as  when  a  wedge  is  driven  into  a  piece  of 
wood,  and  separates  the  parts,  laterally. 

Mrs.  B.  Yes.  The  lateral  pressure  proceeds,  therefore,  en- 
tirely from  the  pressure  downwards,  or  the  weight  of  the  liquid 
above;  and  consequently,  the  lower  the  orifice  is  made  in  the 
vessel,  the  greater  will  be  the  velocity  of  the  water  rushing  out 
of  it.  Here  is  a  vessel  of  water  (fig.  5.),  with  three  stop  cocks 
at  difterent  heights;  we  shall  open  them,  and  you  will  see  with 
what  different  degrees  of  velocity,  the  water  issues  from  them. 
Do  you  understand  this,  Carolinv? 

Caroline.  Oh  yes.  The  water  from  the  upper  spout,  receiv- 
ing but  a  slight  pressure,  on  account  of  its  vicinity  to  the  sur- 
face, flows  but  gently;  the  second  cock,  having  a  greater  weight 
above  it,  the  water  is  forced  out  with  greater  velocity,  whilst 
the  lowest  cock,  being  near  the  bottom  of  the  vessel,  receives 
the  pressure  of  almost  the  whole  body  of  water,  and  rushes  out 
with  the  greatest  impetuosity. 

Mrs.  S.  Very  well;  and  you  must  observe,  that  as  the  late- 
ral pressure,  is  entirely  owine  to  the  pressure  downwards,  it  is 
not  affected  by  the  horizont^  dimensions  of  the  vessel,  which 
contains  the  water,  but  merely  by  its  depth;  for  as  every  particle 
acts  independently  of  the  rest,  it  is  only  the  column  of  particles 
immediately  above  the  orifice,  that  can  weigh  upon,  and  press . 
out  the  water. 

14.  What  results  as  regards  the  pressure  of  fluids?  15.  How  is  this  illus- 
trated by  %.  2,  3,  plate  13?  16.  From  what  does  the  lateral  pre£«iu:e  pro- 
ceed ?  and  to  what  is  it  proportioned,  as  exemplified  in  fig.  5,  plate  13  ? 

Li 


122  MECHANICAL    PROPERTIES    OF    FLUIDS. 

Emily.  The  breadtli  and  widtli  of  the  vessel  then,  can  be  of 
HO  consequence  in  this  respect.  The  lateral  pressure  on  one 
•ide,  in  a  cubical  vessel,  is,  I  suppose,  not  so  great  as  the  pres- 
sure downwards  upon  the  Ijottoni. 

«  Mrs,  B.  *No;  m  a  cubical  vessel,  the  pressure  downwards 
will  be  double  the  lateral  pressure  on  one  side;  for  every  particle 
at  the  bottom  of  the  vessel  is  pressed  upon,  by  a  column  of  the 
whole  depth  of  the  fluid,  whilst  the  lateral  pressure  diminishes 
from  the  bottom  upwards  to  the  surface,  where  the  particles  have 
no  pressure. 

Caroline.  And  from  whence  proceeds  the  pressure  of  fluids 
upwards?  that  seems  to  me  the  most  unaccountable,  as  it  is  in 
direct  opposition  to  gravity. 

Mrs.  n.  And  yet  it  is  in  consequence  of  their  pressure 
downwards.  When,  for  example,  you  pour  water  into  a  tea- 
pot, the  water  rises  in  the  spout,  to  a  level  M'ith  the  water  in  the 
pot.  The  particles  of  water  at  the  bottom  of  the  pot,  are  press- 
ed upon  by  the  particles  above  themj  to  this  pressure  they  will 
yield,  if  there  is  any  mode  of  making  way  for  the  superior  par- 
ticles, and  as  they  cannot  descend,  they  will  change  tlieir  di- 
rection, and  rise  in  the  spout. 

Suppose  the  tea-pot  to  be  filled  with  columns  of  particles  of 
water,  similar  to  that  described  in  fig.  4.,  the  particle  1,  at  the 
bottom,  will  be  pressed  laterally  by  the  particle  2,  and  by  this 
pressure  be  forced  into  the  spout,  where,  meeting  with  the  par- 
ticle 3,  it  presses  it  upwards,  and  this  pressure  will  be  continued 
from  3  to  4,  from  4  to  5,  and  so  on,  till  the  water  in  the  spout, 
has  risen  to  a  level  with  that  in  the  pot. 

Emily.  If  it  were  not  for  this  pressure  upwards,  forcing  the 
water  to  rise  in  the  spout,  the  equilibrium  of  the  fluid  weuld  be 
destroyed. 

Caroline.  True;  but  then  a  tea-pot  is  wide  and  large,  and 
the  weight  of  so  great  a  body  of  water  as  the  pot  will  contain, 
may  easily  force  up  and  support  so  small  a  quantity,  as  will  fill 
the  spout.  But  would  the  same  effect  be  produced,  if  the  spout 
and  tlie  pot,  were  of  equal  dimensions? 

Mrs.  B.  Undoubtedly  it  would.  You  may  even  reverse  the 
experiment,  by  pouring  water  into  the  spout,  and  you  will  find 
that  the  water  will  rise  in  the  pot,  to  a  level  with  that  in  the 
spout;  for  the  pressure  of  the  small  quantity  of  water  in  the 
spot  t,  will  force  up  and  support,  the  larger  quantity  in  the  pot. 

V.  Has  the  extent  of  the  surface  of  a  fluid,  any  effect  upon  its  pressure 
downwards?  18.  What  will  be  the  difference  between  the  pressure  upon 
the  bottom,  and  upon  one  side  of  a  cubical  vessel  ?  19.  What  oco^sions  th« 
upward  pressure,  and  how  is  it  explained  by  fig.  4,  plate  13/ 


MECHANICAL    PROPERTIES    OF    FLUIDS.  123 

In  the  pressure  upwards,  as  well  as  that  laterally,  jou  see  that 
the  force  of  pressure,  depends  entirely  on  the  height,  and  is 
quite  independent  of  the  horizontal  dimensions  of  the  fluid. 

As  a  tea-pot  is  not  transparent,  let  us  try  the  experiment  by 
filling  this  large  glass  goblet,  by  means  of  this  narrow  tube, 
(fig.  6.) 

Caroline.  Look,  Emily,  as  Mrs.  B.^fills  it,  how  the  water 
rises  in  the  goblet,  to  maintain  an  equilibriuJI  with  that  in  the 
tube. 

Now,  Mrs.  B.,  will  you  let  me  fill  the  tube,  by  pouring  water 
into  the  goblet  r 

Mrs.  B.  That  is  impossible.  However,  you  may  try  the 
experiment,  and  I  doubt  not  that  you  will  be  able  to  account 
for  its  failure. 

Caroline.  It  is  very  singular,  that  if  so  small  a  column  of 
water  as  is  contained  in  the  tube,  can  force  up  and  support  the 
whole  contents  of  the  goblet;  that  the  weight  of  all  the  water  in 
the  goblet,  should  not  be  able  to  force  up  the  small  quantity  re- 
quired to  fill  the  tube:— oh,  I  see  now  the  reason,  the  water  in 
the  goblet,  cannot  force  that  in  the  lube  above  its  level,  and  as 
the  end  of  the  tube,  is  considerably  higher  than  the  goblet,  it  can 
never  be  filled  by  pouring  water  into  the  goblet. 

Mrs.  B,  And  if  you  continue  to  pour  water  into  the  goblet 
when  it  is  full,  the  water  will  run  over,  instead  of  rising  above 
its  level  in  the  tube. 

I  shall  now  explain  to  you  the  meaning  of  the  specific  gravity 
of  bodies. 

Caroline.  What!  is  there  another  species  of  gravity,  with 
which  we  are  not  yet  acquainted.^ 

Mrs.  B.  No:  the  specific  gra^dty  of  a  body,  means  simply 
its  weight,  compared  with  that  of  another  body,  of  the  same  size. 
When  we  say,  that  substances,  such  as  lead,  and  stones,  are 
heavy,  and  that  others,  such  as  paper  and  feathers,  are  light, 
we  speak  comparatively;  that  is  to  say,  that  the  first  are  heavy, 
and  the  latter  light,  in  comparison  with  the  generality  of  sub- 
stances in  nature.  Would  you  call  wood,  and  chalk,  light  or 
heavy  bodies? 

Caroline.  Some  kinds  of  wood  are  heavy,  certainly,  as  oak 
and  mahogany;  others  are  light,  as  cedar  and  poplar. 

Emily.  I  think  I  should  call  wood  in  general,  a  heavy  Kody; 
for  cedar  and  poplar,  are  light,  only  in  comparison  to  wood  of  a 
heavier  description.     I  am  at  a  loss  to  determine  whether  chalk 

20.  How  could  the  equilibrium  of  fluids  be  exemplified  by  pouring  water 
in  at  the  spout  of  a-tea-pot  ?  21.  How  by  the  apparatus  represented  at  fig. 
6»  plate  13?  22.  What  is  meant  by  the  specific  gravity  of  a  body?  23.  What 
do  we  in  common  mean  by  calling  a  body  heavy,  or  light  ? 


124  MECHANICAL    PROPERTIES    OF    FLUIDS.  * 

should  be  ranked  as  a  heavy,  or  a  light  body;  I  should  be  inclin- 
ed to  say  the  former,  if  it  was  not  that  it  is  lighter  than  most 
other  minerals.  I  perceive  that  we  have  but  vague  notions  of 
light  and  heavy.  I  wish  there  was  some  standard  of  compari- 
son, to  which  we  could  refer  the  weight  of  all  other  bodies. 

Mrs.  B.  The  necessity  of  such  a  standard,  has  been  so  much 
felt,  that  a  body  has  been  fixed  upon  for  this  purpose.  What 
substance  do  youi^hink  would  be  best  calculated  to  answer  this 
end? 

Caroline,  It  must  be  one  generally  known,  and  easily  obtain- 
ed; lead  or  iron,  for  instance. 

Mrs.  B.  The  metals,  would  not  answer  the  purpose  well,  for 
several  reasons;  they  are  not  always  equally  Compact,  and  they 
are  rarely  quite  pure;  two  pieces  of  iron,  for  instance,  although 
of  the  same  size,  might  not,  from  the  causes  mentioned,  weigh 
exactly  alike. 

Caroline.  But,  Mrs.  B.,  if  you  compare  the  weight,  of  equal 
quantities  of  dift'erent  bodies,  they  will  all  be  alike.  You  know 
the  old  saying,  that  a  pound^of  feathers,  is  as  heavy  as  a  pound 
of  lead.^ 

Mrs.  B.  When  therefore  we  compare  the  weight  of  different 
kinds  of  bodies,  it  would  be  absurd  to  take  quantities  of  equal 
weighty  we  must  take  quantities  of  equal  hulk;  pints  or  quarts, 
not  ounces  or  pounds. 

Caroline.  Very  true;  I  perplexed  myself  by  thinking  that 
quantity  referred  to  weight,  rather  than  to  measure.  It  is  true, 
it  would  be  as  absurd  to  compare  bodies  of  the  same  size,  in  or- 
der to  ascertain  which  was  largest,  as  to  compare  bodies  of  the 
same  weight,  in  order  to  discover  which  was  heaviest. 

Mrs.  B.  In  estimating  the  specific  gravity  of  bodies,  there- 
r'ure,  we  must  compare  equal  bulks,  and  we  shall  find  that  their 
specific  gravity,  will  be  proportional  to  their  weights.  The  body 
which  has  been  adopted  as  a  standard  of  reference,  is  distilled, 
or  rain  water. 

Emily.  I  am  surprised  that  a  fluid  should  have  been  chosen 
tor  this  purpose,  as  it  must  necessarily  be  contained  in  some  ves- 
sel, and  the  weight  of  the  vessel,  will  require  to  be  deducted. 

Mrs.  B.  You  will  find  that  the  comparison  will  be  more 
easily  made  with  a  fluid,  than  with  a  solid;  and  water  you  know 
can  be  every  where  obtained.  In  order  to  learn  the  specific  gra- 
vity of  a  solid  body,  it  is  not  necessary  to  put  a  certain  measure 
of  it  in  one  scale,  and  an  equal  measure  of  water  into  the  other 
scale:  but  simply  to  weigh  the  body  under  trial,  first  in  air,  and 

21.  Why  would  not  the  metals  answer  to  compare  other  bodies  with?  25. 
What  must  be  supposed  equal  in  estimating  the  specific  gravity  of  a  body  ? 
26.  What  has  been  adopted  as  a  standard  for  comparison  ? 


MEeHANICAL    PROPERTIES    OF    FLUIDS.      ,  125 

then  in  water.  If  jou  wei^h  a  piece  of  gold,  in  a  »las9  of  water, 
will  not  the  gold  displace  just  as  much  water,  as  is  equal  to  its 
own  bulk? 

Caroline.  Certainly,  where  one  body  is,  another  cannot  be  at 
the  same  time;  so  that  a  sufficient  quantity  of  water  must  be  re- 
moved, in  order  to  make  way  for  the  gold. 

Mrs.  B.  Yes,  a  cubic  inch  of  water,  to  make  room  for  a  cu- 
bic inch  of  gold;  remember  that  the  bulk,  alone,  is  to  be  consider- 
ed; the  weight,  has  nothing  to  do  with  the  quantity  of  water  dis- 
placed, for  an  inch  of  gold,  does  not  occupy  more  space,  and 
therefore  will  not  displace  more  water,  than  an  inch  ot  ivory,  or 
any  other  substance,  that  \\A\\  sink  in  water. 

Well,  you  will  perhaps  be  surprised  to  hear  that  the  gold  will 
weigh  less  in  water,  than  it  did  out  of  it  ? 

Emily.     And  for  what  reason? 

Mrs.  B.  On  account  of  the  upward  pressure  of  the  particles 
of  water,  which  in  some  measure  supports  the  gold,  and  by  so  do- 
ing, diminishes  its  weight,  j  If  the  body  immersed  in  water,  was 
of  the  same  weight  as  that  fluid,  it  would  be  wholly  supported  by 
it,  just  as  the  water  whicli  it  displaces,  was  supported,  previous  to 
its  making  way  for  the  solid  body.  If  the  body  is  heavier  than 
the  water,  it  cannot  be  wholly  supported  by  it;  but  the  watec 
will  offer  some  resistance  to  its  descent. 

Caroline.  And  the  resistance  which  water  offers  to  the  de- 
scent of  heavy  bodies  immersed  in  it,  (since  it  proceeds  from  the 
upward  pressure  of  the  particles  of  the  fluid,)  must  in  all  cases, 
I  suppose,  be  the  same? 

Mrs.  B.  Yes :  the  resistance  of  the  fluid,  is  proportioned  to  the 
bulk,  and  not  to  the  weight,  of  the  body  immersed  in  it;  all  bodies 
of  the  same  size,  therefore,  lose  the  same  quantity  of  their  weight 
in  water.     Can  you  form  any  idea  what  this  loss  will  be? 

Emily.  I  should  think  it  would  be  equal  to  the  weight  of  the 
water  displaced;  for,  since  that  portion  of  the  water  was  sup- 
ported before  the  immersion  of  the  solid  body,  an  equal  weight 
of  the  solid  body,  will  be  supported. 

Mrs.  B.  You  are  perfectly  right;  a  body  weighed  in  water, 
loses  just  as  much  of  its  weight,  as  is  equal  to  that  of  the  water 
it  displaces;  so  that  if  you  were  to  put  the  water  displaced,  into 
the  scale  to  which  the  body  is  suspended,  it  would  restore  the 
balance. 

You  must  observe,  that  when  you  weigh  a  body  in  water,  in 
order  to  ascertain  its  specific  gravity,  you  must  not  sink  the  dish 
of  tlie  balance  in  the  water;  but  either  suspend  the  body  to  a 

27.  What  is  the  first  step  in  ascertaining  the  specific  gravity  of  a  solid  ? 
28.  What  quantity  of  water  will  the  solid  displace  ?  29.  Why  will  a  solid 
weigh  less  ia  water  than  in  air,  and  to  what  will  the  loss  of  weight  be  equal  ? 

L  2 


126  MECHANICAL    PROPERTIES    OF    FLUIDS. 

hook  at  the  bottom  of  the  dish,  or  else  take  off  the  dish,  and  sus- 
pend to  the  arm  of  the  balance  a  weight  to  counterbalance  the 
other  dish,  and  to  this  attach  the  solid  to  be  weighed,  (fig.  7.)  Now 
suppose  that  a  cubic  inch  of  gold,  weighed  19  ounces  out  of  wa- 
ter, and  lost  one  ounce  of  its  weight  bj  being  weighed  in  water, 
what  would  be  its  specific  gravity? 

Caroline.  The  cubic  inch  of  water  it  displaced,  must  weigh 
that  one  ounces  and  as  a  cubic  inch  of  gold,  weighs  19  ounces, 
gold  is  19  times,  as  heavy  as  water. 

Emily.  I  recollect  having  seen  a  table  of  the  comparative 
weights  of  bodies,  in  which  gold  appeared  to  me  to  be  estimated 
at  19  thousand  times,  the  weiglit  of  water. 

Mrs.  B.  You  misunderstood  the  meaning  of  the  table.  In 
the  estimation  you  allude  to,  the  weight  of  water  was  reckoned 
at  1000.  You  must  observe,  that  Sie  weight  of  a  substance 
when  not  compared  to  that  of  any  other,  is  perfectly  arbitrary; 
and  when  water  is  adopted  as  a  standard,  we  may  denominate 
its  weight  by  any  number  we  please;  but  then  the  weight  of  all 
bodies  tried  by  this  standard,  must  be  signified  by  proportional 
numbers. 

Carolitie.  We  may  call  the  weight  of  water,  for  example,  on6, 
and  then  that  of  gold,  would  be  nineteen;  or  if  we  choose  to  call 
the  weight  of  water  1000,  that  of  gold  would  be  19,000.  In 
short,  specific  gravity,  means  how  many  times  more  a  body 
weighs,  than  an  equal  bulk  of  water. 

Mrs.  B.  It  is  rather  the  weight  of  a  body  compared  with  a 
portion  of  water  equal  to  it  in  bulk;  for  the  specific  gravity  of 
hiany  substances,  is  less  than  that  of  water. 

Caroline.  Then  you  cannot  ascertain  the  specific  gravity  of 
audi  substances,  in  the  same  manner  as  that  of  gold;  \ov  a  body 
that  is  lighter  than  water,  will  float  on  its  surface,  without  dis- 
placing any  of  it. 

Mrs.  B.  If  a  body  were  absolutely  without  weight,  it  is  true 
that  it  would  not  displace  a  drop  of  water,  but  the  bodies  we  are 
treating  of,  have  all  some  weight,  however  small;  and  will,  there- 
fore, displace  some  quantity.  If  the  body  be  lighter  than  wa- 
ter, it  will  not  sink  to  a  level  with  its  surface,  and  therefore  it 
will  not  displace  so  much  water  as  is  equal  to  its  bulk;  but  only 
<s(X  much,  as  is  equal  to  its  weight.  A  ship,  you  must  have  ob- 
served, sinks  to  some  depth  in  water,  and  the  heavier  it  is  laden, 
the  deeper  it  sinks,  as  it  always  displaces  a  quantity  of  water, 
equal  to  its  own  weight. 

30.  What  is  the  arrangement  represented  by  fig.  7,  plate  13?  31.  What 
is  stated  of  gold  as  an  example  ?  32.  In  comparing  a  body  with  water,  this 
is  sometimes  called  1000,  what  must  be  observed  ?  33.  What  quantity  of 
water  is  displaced,  by  a  body  floating  upon  its  surface  ? 


MECHANICAL    PROPERTIES    OF    FLUIDS.  127 

Caroline.  But  you  said  just  now,  that  in  the  immersion  of 
gold,  the  bulk,  and  not  the  weight  of  body,  was  to  be  con- 
sidered. 

Mrs.  B.  That  is  the  case  with  all  substances  which  are  hea- 
vier than  water;  but  since  those  which  are  lighter,  do  not  dis- 
place so  much  as  their  own  bulk,  the  quantity  they  displace  is 
not  a  test  of  their  specific  gravity. 

In  order  to  obtain  the  specific  gravity  of  a  body  which  is  lighter 
than  water,  you  must  attach  to"*  it  a  heavy  one,  whose  specific 
gravity  is  known,  and  immerse  them  together;  the  specific  gra- 
vity of  the  lighter  body,  may  then  be  easily  calculated  from  ob- 
serving the  loss  of  weight  it  produces,  in  the  heavy  body. 

Ermly.  But  are  there  not  some  bodies  which  have  exactly 
ihe  same  specific  gravity  as  water? 

Mrs.  B.  Undoubtedly;  and  (such  bodies  will  remain  at  rest 
in  whatever  situation  they  are  placed  in  water.;  Here  is  a  piece 
of  wood  which  I  have  procured,  because  it  is  of  a  kind  which  is 
precisely  the  weight  of  an  equal  bulk  of  v/ater;  in  whatever 
part  of  this  vessel  of  water  you  place  it,  you  will  find  that  it  will 
remain  stationary. 

Caroline.  I  shall  first  put  it  at  the  bottom;  from  thence,  of 
course,  it  cannot  rise,  because  it  is  not  lighter  than  water.  Now 
I  shall  place  it  in  the  middle  of  the  vessel;  it  neither  rises  nor 
sinks,  because  it  is  neither  lighter  nor  heavier  than  the  water. 
Now  I  will  lay  it  on  the  surface  of  the  water ;  but  there  it  sinks 
a  little — what  is  the  reason  of  that,  Mrs.  B.  ? 

Mrs.  B.  Since  it  is  not  lighter  than  the  water,  it  cannot  float 
upon  its  surface;  since  it  is  not  heavier  than  water,  it  cannot 
sink  below  its  surface:  it  will  sink  therefore,  onl^^  till  the  upper 
surface  of  both  bodies  are  on  a  level,  so  that  the  piece  of  wood  is 
just  covered  with  water.  If  you  poured  a  few  drops  of  water 
into  the  vessel,  (so  gently  as  not  to  give  them  momentum)  they 
would  mix  with  the  water  at  the  surface,  and  not  sink  lower. 

Caroline.  I  now  understand  the  reason,  why,  in  drawing 
up  a  bucket  of  water  out  of  a  well,  the  bucket  feels  so  much  hea- 
vier when  it  rises  above  the  surface  of  the  water  in  the  well;  for 
whilst  you  raise  it  in  the  water,  the  water  within  the  bucket  be- 
ing of  the  same  specific  gravity  as  the  water  on  the  outside,  will 
be  whoUy'supportedby  the  upward  pressure  of  the  water  beneath 
the  bucket,  and  consequently  very  little  force  will  be  required 
to  raise  it;  but  as  soon  as  tne  bucket  rises  to  the  surface  of  the 
well,  you  immediately  perceive  the  increase  of  weight. 

34  How  can  you  find  the  specific  gravity  of,  a  solid  which  is  lighter  thau 
water  ?  35.  What  is  observed  of  a  body  whose  specific  gravity  is  the  same 
as  that  of  water  ?  36.  What  is  the  reason  that  in  drawing  a  bucket  of  water 
from  a  well,  its  weight  is  not  perceived  until  it  rises  above  the  surface  r 


128  OF    SPRINGS,   rOUNTAINS,   &C. 

Emily.  And  how  do  you  ascertain  the  specific  gravity  of 
fluids? 

Mrs.  B.  By  means  of  an  hydrometer;  this  instrument  is 
made  of  various  materials,  and  in  different  forms,  one  of  which  I 
will  show  you.  It  consists  of  a  thin  brass  ball  A,  (fig.  8,  plate 
13.)  with  a  graduated  tube  B,  and  the  specific  gravity  of  the  li- 
quid, is  estimated  by  the  depth  to  which  the  instrument  sinks  in  it, 
or  by  the  weight  required  to  sink  it  to  a  given  depth.  There  is 
a  small  bucket  C,  suspended  at  the  lower  end,  and  also  a  little 
dish  on  the  graduated  tube;  into  either  of  these,  small  weights 
may  be  put,  until  the  instrument  sinks  in  the  fluid,  to  a  mark  on 
the  tube  B;  the  amount  of  weight  necessary  for  this,  will  enable 
you  to  discover  the  specific  gravity  of  the  fluid^ 

I  must  now  take  leave  of  you;  but  there  remain  yipt  many  ob- 
servations to  be  made  on  fluids:  we  shall,  therefore,  resume  this 
subject  at  our  next  interview. 

37.  Describe  the  instrument  represented  by  fig.  8,  plate  13,  and  also  how, 
and  for  what  it  is  used  ? 


CONVERSATION  XI. 


OF  SPRINGS,  FOUNTAINS,  &o. 

•B  THE  ASCENT  OF  VAPOUR  AND  THE  FORMATION  OF  CLOTTDS. — OF  THE 
FORMATION  AND  FALL  OF  RAIN,  &C. — OF  THE  FORMATION  OF  SPRINGS. 
•F  RIVERS  AND  LAKES. — OF  FOUNTAINS. 

CAROLINE. 

There  is  a  question  I  am  very  desirous  of  asking  you,  respect- 
ing fluids,  Mrs.  B.,  which  has  often  perplexed  me.  What  is  the 
leason  that  the  great  quantity  of  rain  which  falls  upon  the  earth 
and  sinks  into  it,  does  not,  in  the  course  of  time,  injure  its  solid- 
ity? The  sun  and  the  wind,  I  know,  dry  the  surface,  but  they 
have  no  effect  on  the  interior  parts,  where  there  must  be  a  pro- 
digious accumulation  of  moisture. 

Mrs.  B.  Do  you  not  know,  that,  in  the  course  of  time,  all  the 
water  which  sinks  into  the  ground,  rises  out  of  it  again?  It  is  the 


OF    SPRINGS,    FOUNTAINS,    &C.  129 

same  water  which  successively  forms  seas,  rivers,  springs,  clouds, 
rain,  and  sometimes  hail,  snow  and  ice.  >  If  you  will  take  the 
taouble  of  following  it  through  these  various  changes,  you  will  un- 
derstand why  the  earth  is  not  yet  drowned,  by  the  quantity  of 
water  which  has  fallen  upon  it,  since  its  creation;  and  you  will 
even  be  convinced,  that  it  does  not  contain  a  single  drop  more 
water  now,  than  it  did  at  that  period. 

Let  us  consider  how  the  clouds  were  originally  formed.  When 
the  first  rays  of  the  sun  warmed  the  surface  of  the  earth,  the 
heat,  by  separating  the  particles  of  water,  rendered  them  lighter 
than  the  air.  This,  you  know,  is  the  case  with  steam  or  vapour. 
What  then  ensues? 

Caroline.  When  lighter  than  the  air,  it  will  naturally  rise; 
and  now  I  recollect  your  telling  us  in  a  preceding  lesson,  that 
the  heat  of  the  sun  transformed  the  particles  of  water  into  va- 
pour; in  consequence  of  which,  it  ascended  into  the  atmosphere^ 
where  it  formed  clouds. 

Mrs.  B.  We  have  then  alreadjr  followed  water  through  two 
of  its  transformations;  from  water  it  becomes  vapour,  and  from 
vapour  clouds. 

Emily.  But  since  this  watery  vapour  is  lighter  than  the  air, 
why  does  it  not  continue  to  rise;  and  why  does  it  unite  again,  to 
form  clouds? 

Mrs.  B.  Because  the  atmosphere  diminishes  in  density,  as 
it  is  more  distant  from  the  earth.  The  vapour,  therefore,  which 
the  sun  causes  to  exhale,  not  only  from  seas,  rivers,  and  lakes, 
but  likewise  from  the  moisture  on  the  land,  rises  till  it  reaches 
a  region  of  air  of  its  own  specific  gravity;  and  there,  you  know, 
it  will  remain  stationary.  By  the  frequent  accession  of  fresh 
vapour,  it  gradually  accumulates,  so  as  to  forni  those  large  bo- 
dies of  vapour,  which  we  call  clouds:  and  the  particles,  at  length 
unitinff,  become  too  heavy  for  the  air  to  support,  and  fall  to  the 
ground. 

Caroline.  They  do  fall  to  the  ground,  certainly,  when  it 
rains;  but,  accord.ing  to  your  theory,  I  should  have  imagined, 
that  when  the  clouds  became  too  heavy,  for  the  region  of  air  in 
which  they  were  situated,  to  support  them,  they  would  descend, 
till  they  reached  a  stratum  of  air  of  their  own  weight,  .and  not 
fall  to  the  earth;  for  as  clouds  are  formed  of  vapour,  they  can- 
not be  so  heavy  as  the  lowest  regions  of  the  atmosphere,  other- 
wise the  vapour  would  not  have  risen. 

Mrs.  B.  If  you  examine  the  manner  in  which  the  clouds 
descend,  it  will  obviate  this  objection.     In  falling,  several  of  the 

1 .  Why  do  not  the  frequent  rains,  fill  the  eeirth  with  water  ?  S.  Why  will 
vapour  rise  ?  to  what  height  will  it  ascend,  and  what  will  it  form  ?  3.  How 
may  drops  of  rain  be  formed  ? 


136  OF    SPRINGS,    FOUNTAINS,    &C. 

watery  particles  come  within  the  sphere  of  each  other's  attrac- 
tion, and  unite  in  the  form  of  a  drop  of  water.  The  vapour 
thus  transformed  into  a  shower,  is  heavier  than  any  part  of  the 
atmosphere,  and  consequently  descends  to  the  earth. 

Caroline,     How  v/onderfully  curious! 

Mrs.  B.  It  is  impossible  to  consider  any  part  of  nature  at- 
tentively, without  being  struck  with  admiration  at  the  wisdom  it 
displays;  and  I  hope  you  will  never  contemplate  these  wonders, 
without  feeling  your  heart  glow  with  admiration  and  gratitude, 
towards  their  bounteous  Author.  Observe,  that  if  the  waters 
were  never  drawn  out  of  the  earth,  all  vegetation  would  be  de- 
stroyed by  the  excess  of  moisture;  if,  on  the  other  hand,  tlie 
plants  were  not  nourished  and  refreshed  by  occasional  showers, 
the  drought  would  be  equally  fatal  to  them.  If  the  clouds  con- 
stantly remained  in  a  state  of  vapour,  they  might,  as  you  re- 
marked, descend  into  a  heavier  stratum  of  the  atmosphere,  but 
could  never  fall  to  the  ground;  or  were  the  power  of  attraction 
more  than  sufficient  to  convert  the  vapour  into  drops,  it  would 
transform  the  cloud  into  a  mass  of  water,  which,  instead  of 
nourishing,  would  destroy  the  produce  of  the  earth. 

Water  then  ascends  in  the  form  of  vapour,  and  descends  in 
that  of  rain,  snow,  or  hail,  all  of  which  ultimately  become  water. 
Some  of  this  falls  into  the  various  bodies  of  water  on  the  sur- 
face of  the  globe,  the  remainder  upon  the  land.  Of  the  latter, 
part  reascends  in  the  form  of  vapour,  part  is  absorbed  by  the 
roots  of  vegetables,  and  part  descends  into  the  earth,  where  it 
forms  springs. 

Entity.  Is  there  then  no  difference  between  rain  water,  and 
spring  water  ? 

Mrs,  B.  They  are  originally  the  same;  but  that  portion  of 
rain  water  which  goes  to  supply  springs,  dissolves  a  number  of 
foreign  particles,  which  it  meets  with  m  its  passage  through  the 
various  soils  it  traverses. 

Caroline.  Yet  spring  water  is  more  pleasant  to  the  taste, 
appears  more  transparent,  and,  I  should  have  supposed,  would 
have  been  more  pure  than  rain  water. 

Mrs.  B.  No;  excepting  distilled  water,  rain  water  is  the 
most  pure  we  can  obtain;  it  is  its  purity  which  renders  it 
insipia;  whilst  the  various  salts  and  diiferent  ingredients,  dis- 
solved in  spring  water,  give  it  a  species  of  flavour,  which  habit 
renders  agreeable;  these  salts  do  not,  in  any  degree,  aft'ect  its 
transparency;  and  the  filtration  it  undergoes,  through  gravel  and 

4.  What  becomes  of  the  water  after  it  has  fallen  to  the  earth  ?  5.  What 
is  the  difference  between  rain  water,  and  that  from  springs  ?  6.  Why  is  rain 
more  pure  than  spring  water  ?  7.  Why  i«  spring  water  more  agreeable  to 
the  palate  ? 


OF    SPRINGS,    FOUNTAINS,    &,C.  151 

sand,  cleanses  it  from  all  foreign  matter,  which  it  has  not  the 
power  of  dissolving. 

Emily,  How  is  it  that  the  rain  water  does  not  continue  to 
descend  by  its  gravity,  instead  of  collecting  together,  and  form- 
ing springs? 

Mrs,  B.  When  rain  falls  on  the  surface  of  the  earth,  it 
continues  making  its  way  downwards  through  tlie  pores  and 
crevices  in  the  ground.  When  several  drops  meet  in  their  sub- 
terraneous passage,  they  unite  and  form  a  little  rivulet;  this,  in 
its  progress,  meets  with  otlier  rivulets  of  a  similar  description, 
and  they  pursue  their  course  together  within  the  eartli,  till  they 
are  stopped  by  some  substance,  such  as  rock,  or  clay,  which 
they  cannot  penetrate. 

Caroline.  But  you  say  that  there  is  some  reason  to  believe 
that  water  can  penetrate  even  the  pores  of  gold,  and  it  cannot 
meet  with  a  substance  more  dense? 

Mrs.  B.  But  if  water  penetrate  the  pores  of  gold,  it  is  only 
when  under  a  strong  compressive  force,  as  in  the  Florentine 
experiment;  now  in  its  passage  towards  the  centre  of  tlie  earth, 
it  IS  acted  upon  by  no  other  power  than  gravity,  which  is  not 
sufficient  to  make  it  force  its  way,  even  through  a  stratum  of 
clay.  This  species  of  earth,  though  not  remarkably  dense,  be- 
ing of  great  tenacity,  will  not  admit  the  particles  of  water  to 
pass.  When  water  encounters  any  substance  of  this  nature, 
therefore,  its  progress  is  stopped,  and  it  is  diffused  through  the 
porous  earth,  and  sometimes  the  pressure  of  the  accumulating 
waters,  forms  a  bed,  or  reservoir.  This  will  be  more  clearly 
explained  by  fio-.  9,  plate  13,  which  represents  a  section,  of  the 
interior  of  a  hill  or  mountain.  A,  is  a  body  of  water,  such  as  I 
have  described,  which,  when  filled  up  as  high  as  B,  (by  the  con- 
tinual accession  of  water  it  receives  from  the  ducts  or  rivulets 
«,  «,  «,  a,)  finds  a  passage  out  of  the  cavity,  and,  impelled  by 
gravity,  it  runs  on,  till  it  makes  its  way  out  of  the  ground  at  the 
side  of  the  hill,  and  there  forms  a  spring,  C. 

Caroline,  Gravity  impels  downwards  towards  the  centre  of 
the  earth;  and  the  spiing  in  this  figure  runs  in  an  horizontal 
direction. 

Mrs.  B.  Not  entirely.  There  is  some  declivity  from  the 
reservoir,  to  the  spot  where  the  water  issues  out  of  the  ground; 
and  gravity,  you  know,  will  bring  bodies  down  an  inclined  plane, 
as  well  as  in  a  perpendicular  direction. 

Caroline.  But  though  the  spring  may  descend,  on  first  issu- 
ing, it  must  afterwards  rise  to  reach  tne  surface  of  the  earth; 
and  that  is  in  direct  opposition  to  gravity. 

8.  What  causes  the  water  to  collect  and  form  springs  ?  9.  Why  cannot 
water  penetrate  through  clay  ?    10.  What  is  represented  by  fig.  9,  plate  13? 


1S£  OF    SPRINGS,    FOUNTAINS,    &C. 

Mrs.  B.  A  spring  can  never  rise  above  the  level  of  the  re- 
servoir whence  it  issues;  it  must,  therefore,  find  a  passage  to 
some  part  of  the  surface  of  the  earth,  that  is  lower,  or  nearer  the 
centre,  tlian  the  reservoir.  It  is  true  that,  in  this  figure,  the 
spring  rises  in  its  passage  from  B  to  C;  but  this,  I  think,  witli 
a  little  reflection,  jou  will  be  able  to  account  for. 

Emily,  Oh,  jes;  it  is  owing  to  the  pressure  of  fluids  up- 
wards; and  the  water  ri<«es  in  the  duct,  upon  the  same  principle 
as  it  rises  in  the  spout  of  a  tea-pot;  that  is  to  say,  in  order  to 
preserve  an  equilibrium  with  the  water  in  tlie  reservoir.  Now 
1  think  I  understand  the  nature  of  springs:  the  water  will  flow 
through  a  duct,  whether  ascending  or  descending,  provided  it 
never  rises  higher  than  the  reservoir. 

Mrs.  B.  Water  may  thus  be  conveyed  to  every  part  of  a 
town,  and  to  the  upper  part  of  the  houses,  if  it  is  originally 
brought  from  a  height,  superior  to  any  to  which  it  is  conveyed. 
Have  you  never  observed,  when  the  pavements  of  the  streets 
have  been  mending,  the  pipes  which  serve  as  ducts  for  the  con- 
veyance of  the  water  through  the  town? 

Emily.  Yes,  frequently;  and  I  l\ave  remarked  that  when 
any  of  these  pipes  have  been  opened,  the  water  rushes  upwards 
from  them,  with  great  velocity;  which,  I  suppose,  proceecls  from 
the  pressure  of  the  water  in  the  reservoir,  which  forces  it  out. 

Caroline.  I  recollect  having  once  seen  a  very  curious  glass, 
called  Tantalus's  cup;  it  consists  of  a  goblet,  containing  a  small 
figure  of  a  man,  and  whatever  quantity  of  water  you  pour  into 
the  goblet,  it  never  rises  higlier  than  the  breast  of  the  figure.  Do 
you  know  how  that  is  contrived? 

Mrs.  B.  It  is  by  means  of  a  s^'phon,  or  bent  tube,  which  is 
concealed  in  the  body  of  the  figure.  This  tube  rises  through  one 
of  the  less,  as  high  as  the  breast,  and  there  turning,  descends 
through  the  other  leg,  and  from  thence  through  the  foot  of  the 
goblet,  where  the  water  runs  out.  (fi^.  1,  plate  14.)  When  you 
pour  water  into  the  glass  A,  it  must  rise  in  the  syphon  B,  in  pro- 
portion as  it  rises  in  the  glass;  and  when  the  glass  is  filled  to  a 
level  with  the  upper  part  of  the  syphon,  the  water  will  run  out 
through  the  other  leg  of  the  figure,  and  will  continue  running 
out,  as  fast  as  you  pour  it  in;  therefore  the  glass  can  never  fill 
any  higher. 

Emily.  I  think  the  new  well  that  has  been  made  at  our 
country-house,  must  be  of  that  nature.  We  had  a  great  scar- 
city of  water,  and  my  father  has  been  at  considerable  expense  to 
dig  a  well;  after  penetrating  to  a  great  depth,  before  water  could 

11.  How  can  you  account  for  its  rising  upwards,  as  represented  at  C  ? 
12.  In  conveying  water  by  means  of  pipes,  how  must  the  reservoir  be  situat- 
ed? 13.  What  is  the  instrument  called,  which  is  represented  in  Dlate  14, 
%.  1, — and  how  does  it  operate  ? 


m 


n 


Oy    SPRINGS,    FOUNTAINS,    &C.  13S 

be  found,  a  spring  was  at  length  discovered,  but  the  water  rose 
only  a  few  feet  above  the  bottom  of  the  well 5  and  sometimes  it 
is  quite  dry. 

Mrs.  B,  This  has,  however,  no  analogy  to  Tantalus's  cupj 
but  is  owing  to  the  very  elevated  situation  of  your  country- 
house. 

Emily.  I  believe  I  guess  the  reason.  There  cannot  be  a  re- 
servoir of  water  near  the  summit  of  a  hill;  as  in  such  a  situation, 
there  will  not  be  a  sufficient  number  of  rivulets  formed,  to  supply- 
one;  and  witliout  a  reservoir,  there  can  be  no  spring.  In  such 
situations,  therefore,  it  is  necessary  to  dig  very  deep,  in  order  to 
meet  with  a  spring;  and  when  we  give  it  vent,  it  can  rise  only 
as  high  as  the  reservoir  from  whence  it  flows,  which  will  be  but 
little,  as  the  reservoir  must  be  situated  at  some  considerable 
depth  below  the  summit  of  the  hill. 

Caroline.  Your  explanation  appears  very  clear  and  satisfac- 
tory; but  I  can  contradict  it  from  experience.  At  the  very  top 
of  a  hill,  near  our  country-house,  there  is  a  large  pond,  and,  ac- 
cording to  your  theory,  it  would  be  impossible  there  should  be 
springs  in  such  a  situation  to  supply  it  with  water.  Then  you 
know  that  I  have  crossed  the  Alps,  and  I  can  assure  you,  that 
tliere  is  a  fine  lake  on  the  summit  of  Mount  Cenis,  the  highest 
mountain  we  passed  over. 

Mrs.  B.  Were  there  a  lake  on  the  summit  of  Mount  Blanc, 
which  is  the  highest  of  the  Alps,  it  would  indeed  be  wonderful. 
But  that  on  Mount  Cenis,  is  not  at  all  contradictory  to  our  the- 
ory of  springs;  for  this  mountain  is  surrounded  by  others,  much 
more  elevated,  and  the  springs  which  feed  the  lake  must  descend 
from  reservoirs  of  water,  formed  in  those  mountains.  This  must 
also  be  the  case  with  the  pond  on  the  top  of  the  hill;  there  is 
doubtless,  some  more  considerable  hill  in  the  neighbourhood, 
which  supplies  it  with  water. 

Emily,  I  comprehend  perfectly,  why  the  water  in  our  well 
never  rises  high:  but  I  do  not  understand  why  it  should  occa- 
sionally be  dry. 

Mrs.  B.  Because  the  reservoir  from  which  it  flows,  being  i* 
an  elevated  situation,  is  but  scantily  supplied  with  water;  after 
a  long  drought,  therefore,  it  may  be  drained,  and  tlie  spring  dry, 
till  the  reservoir  be  replenished  by  fresh  rains.  It  is  not  un- 
common to  see  springs  flow  with  great  violence  in  wet  seasons, 
which  at  other  times,  are  perfectly  dry. 

Caroline.    But  tliere  is  a  spring  in  our  grounds,  which  more 

14.  Why  are  wells  rarely  well  supplied  with  water,  in  elevated  situations  ? 
15.  When  water  is  found  in  elevated  situations,  whence  is  it  supplied?  16. 
Wells  and  springs,  at  some  periods  well  supplied,  fail  at  otherB ;  how  is  thii  ac- 
counted for? 

M 


1S4  OF    SPRINGS,    FOUNTAINS,   &C. 

frequently  flows  in  dry,  than  in  wet  w^atherj  how  is  that  to  be 
accounted  for? 

Mrs.  B.  The  spring,  probabl}^,  comes  from  a  reservoir  at  a 
great  distance,  and  situated  very  deep  in  the  ground:  it  is, 
therefore,  some  length  of  time  before  tlie  rain  reaches  the  reser^ 
Toirj  and  another  considerable  portion  must  elapse,  whilst  the 
water  is  making  its  way,  from  the  reservoir,  to  the  surface  of  the 
earthj  so  that  the  dry  Aveather  may  probably  have  succeeded  the 
rains,  before  the  spring  begins  to  flow;  and  the  reservoir  may  be 
exhausted,  by  the  time  the  wet  weatlier  sets  in  again. 

Caroline.  1  doubt  not  but  this  is  the  case,  as  the  spring  is  in 
a  very  low  situation,  therefore,  the  reservoir  may  be  at  a  great 
distance  from  it. 

Mrs.  B.  Springs  which  do  not  constantly  flow,  are  called 
intermitting,  and  are  occasioned  by  tlie  reservoir  being  imper- 
fectly supplied.  Independently  of  the  situation,  this  is  always 
the  case,  when  the  duct,  or  ducts,  which  convey  the  water  into  the 
reservoir,  are  smaller  than  those  which  carry  it  off*. 

Caroline.  If  it  runs  out,  faster  than  it  runs  in,  it  will  of  course 
sometimes  be  empty.  Do  not  rivers  also,  derive  their  source  from 
springs.^ 

Mrs.  B.  Yes,  they  generally  take  their  source  in  mountain- 
ous countries,  where  springs  are  most  abundant. 

Caroline.  I  understood  you  that  springs  were  more  rare,  in 
elevated  situations. 

Mrs.  B.  You  do  not  consider  that  mountainous  countries, 
abound  equally  witli  high,  and  low  situations.  Reservoirs  of  wa- 
ter, wliich  are  formed  in  the  bosoms  of  mountains,  generally  find 
a  vent,  eitlier  on  their  declivity,  or  in  the  valley  beneath;  while 
subterraneous  reservoirs,  formed  in  a  plain,  can  seldom  find  a 
passage  to  tlie  surface  of  the  earth,  but  remain  concealed,  unless 
discovered  by  digging  a  well.  When  a  spring  once  issues  at  the 
surface  of  the  earth,  it  continues  its  course  externally,  seeking 
always  a  lower  ground,  for  it  can  no  longer  rise. 

Emily.  Then  what  is  the  consequence,  if  the  spring,  or,  as  1 
should  now  ratlier  call  itf  the  rivulet,  runs  into  a  situation,  which 
is  surrounded  by  higlier  ground.^ 

Mrs.  B.  Its  course  is  stopped;  the  water  accumulates,  and 
it  forms  a  pool,  pond,  or  lake,  according  to  the  dimensions  of 
the  body  of  water.  The  lake  of  Geneva,  in  all  probability,  owes 
its  origin  to  the  Rhone,  which  passes  through  it:  if,  when  this 
river  first  entered  the  valley,  which  now  forms  the  bed  of  the 

17.  Some  springs  flow  abundantly  in  dry  weather,  which  occasionally  fail 
in  wet  weather,  how  may  this  be  explained?  18.  What  is  meant  by  inter- 
niittin*  springs  ?  19.  Whence  do  rivers,  in  general,  derive  their  water  ?  20. 
Why  do  sjH-iiigs  abound  more  in  mountainous,  than  in  level  countries 


OF    SPRINGS,   FOUNTAINS,  &C.  135 

Lake,  it  found  itself  surrounded  by  higher  grounds,  its  waters 
would  there  accumulate,  till  they  rose  to  a  level  with  that  part 
of  the  valley,  wliere  the  Rhone  now  continues  its  course  beyond 
the  Lake,  and  from  whence  it  flows  through  valleys,  occasionally 
forming  other  small  lakes,  till  it  reaches  the  sea. 

Emily.     And  are  not  fountains,  of  the  nature  of  springs? 

Mrs.  B.  Exactly*  A  fountain  is  conducted  perpendicularly 
upwards,  by  the  spout  or  adjutage  A,  through  which  it  flows; 
and  it  will  rise  nearly  as  high  as  the  reservoir  B,  from  whence  it 
proceeds.     (Plate  14.  fig.  2.) 

Caroline.     Why  not  quite  as  hij^h? 

Mrs.  B.  Because  it  meets  with  resistance  from  the  air,  in 
its  ascent;  and  its  motion  is  impeded  by  friction  against  the 
!»pout,  where  it  rushes  out. 

Emily.  But  if  the  tube  through  which  the  water  rises  be 
smooth,  can  there  be  any  friction?  especially  with  a  fluid,  whose 
particles  yield  to  the  slightest  impression. 

Mrs.  B.  Friction,  (as  we  observed  in  a  former  lesson,)  may 
be  diminished  by  polishing,  but  can  never  be  entirely  destroyed; 
and  though  fluicls,  are  less  susceptible  of  friction,  than  solid  bo- 
dies, they  are  still  affected  by  it.  Another  reason  why  a  foun- 
tain will  not  rise  so  high  as  its  reservoir,  is,  that  as  all  the  water 
which  spouts  up,  has  to  descend  again,  it  in  doing  so,  presses,  or 
strikes  against  the  under  parts,  and  forces  them  sideways,  spread- 
ing the  column  into  a  head,  and  rendering  it  botli  wider,  and 
shorter,  than  it  otherwise  would  be. 

At  our  next  meeting,  we  shall  examine  the  mechanical  pro- 
perties of  the  au',  which  being  an  elastic  fluid,  differs  in  many 
respects,  from  liquids. 

21.  How  are  lakes  formed?  22.  What  causes  water  to  rise  in  fountain?, 
and  how  is  this  explained  by  figure  2,  plate  14  ?  23,  Why  will  not  the  foun- 
tain rise  to  the  height  of  the  water  in  the  reservoir  ? 


CONVERSATION  XIL 


ON  THE  MECHANICAL  PROPERTIES  OF  AIR. 

*F  THE  SPRING  OR   ELASTICITY  OF  THE  AIR. — OF  THE  WEIGHT   OF  THE 
AIR. — EXPERIMENTS  WITH    THE   AIR  PUMP. — OF    THE  BAROMETER. — 

MODE  OF  WEIGHING    AIR. — SPECIFIC   GRAVITY    OF   AIR. OF    PUMPS. 

DESCRIPTION   OF  THE  SUCKING  PUMP. — DESCRIPTION  OF  THE  FORCING 
PUMP. 

MRS.    B. 

At  our  last  meeting  we  examined  the  properties  of  fluids  in 
*^neral,  and  more  particularly  of  such  as  are  called  non-elastic 
fluids,  or  liquids. 

There  is  another  class  of  fluids,  distinguished  by  the  name  of 
aeriform,  or  elastic  fluids,  the  principal  of  which  is  the  air  we 
breathe,  which  surrounds  the  earth,  and  is  called  the  atmo- 
sphere. 

Emily,  There  are  then  other  kinds  of  air,  besides  the  atmo- 
sphere? 

Mrs.  B.  Yes;  a  great  variety;  butf^they  differ  only  in  their 
chemical,  and  not  in  their  mechanical  properties;  and  as  it  is  the 
latter  we  are  to  examine,  w^e  shall  not  at  present  inquire  into 
their  composition,  but  confine  our  attention  to  the  mechanical 
properties  of  elastic  fluids  in  general. 

Caroline.  And  from  whence  arises  this  difference,  between 
elastic,  and  non-elastic  fluids? 

Mrs.  B.  There  is  no  attraction  of  cohesion,  between  the  par- 
ticles of  elastic  fluids;  so  that  the  expansive  power  of  heat,  has  no 
adversary  to  contend  v.'ith,  but  gravity;  any  increase  of  tempera- 
ture, therefore,  expands  elastic  fluids  considerably,  and  a  dimi- 
nution, proportionally  condenses  them. 

The  most  essential  point,  in  which  air,  differs  from  other  fluids, 
is  in  its  spring  or  elasticity;  that  is  to  say,  its  power  of  increas- 
ing, or  diminishing  in  bulk,  accordingly  as  it  is  more,  or  less,  com- 
pressed: a  power  of  which  I  have  informed  you,  liquids  are  al- 
most wholly  deprived. 

1.  Into  what  two  kinds  are  fluids  divided.''  2.  There  are  different  kinds 
of  elastic  fluids,  in  what  properties  are  they  alike,  and  in  what  do  they  dif- 
fer ?  3.  In  what  particular  do  elastic,  differ  from  non-elastic,  fluids .'  4. 
"What  is  meant  by  Uie  elasticity  of  air  ^ 


M^ECHANICAL  PROPERTIES  OF  AIR.  137 

Emily.  I  think  I  understand  the  elasticity  of  the  air  very 
well  from  what  you  formerly  said  of  it;  but  what  perplexes  me  is, 
its  having  gravity;  if  it  is  heavy,  and  we  are  surrounded  by  it, 
why  do  we  not  feel  its  weight? 

Caroline.  It  must  be  impossible  to  be  sensible  of  the  weight 
of  such  infinitely  small  particles,  as  those  of  wliich  the  air  is 
composed:  particles  which  are  too  small  to  be  seen,  must  be  too 
light  to  be  felt. 

Mrs.  B.  You  are  mistaken,  my  dear;  the  air  is  mucli  heavier 
than  you  imagine;  it  is  true,  that  the  particles  which  compose  it, 
are  small;  but  then,  reflect  on  their  quantity:  the  atmosphere 
extends  in  height,  a  great  number  of  miles  from  the  earth,  and 
its  gravity  is  such,  that  a  man  of  middling  stature,  is  computed 
(when  the  air  is  heaviest)  to  sustain  the  weight  of  about  14  tons. 

Caroline.  Is  it  possible!  I  should  have  thought  such  a  weight 
would  have  crushed  any  one  to  atoms. 

Mrs.  B.  That  would,  indeed,  be  the  case,  if  it  were  not  for 
the  equality  of  the  pressure,  on  every  part  of  the  body;  but  when 
thus  diftused,  we  can  bear  even  a  much  greater  weight,  without 
any  considerable  inconvenience.  In  bathing  we  support  the 
weight  and  pressure  of  the  water,  in  addition  to  that  of  tlie  atmo- 
sphere; but  because  this  pressure  is  equally  distributed  over  the 
body,  we  are  scarcely  sensible  of  it;  whilst  if  your  shoulders, 
your  head,  or  any  particular  part  of  your  frame,  were  loaded  with 
the  additional  weight  of  a  hundred  pounds,  you  would  soon  sink 
under  the  fatigue.  Besides  this,  our  bodies  contain  air,  the  spring 
of  which,  counterbalances  the  weight  of  tlie  external  air,  and  ren- 
ders us  insensible  of  its  pressure. 

Caroline.  But  if  it  were  possible  to  relieve  me  from  the  weight 
of  the  atmosphere,  should  I  not  feel  more  light  and  agile? 

Mrs.  B.  On  the  contrary,  the  air  within  you,  meeting  with 
no  external  pressure  to  restrain  its  elasticity,  would  distend 
your  body,  and  at  length  bursting  some  of  tlie  parts  which  con- 
fined it,  put  a  period  to  your  existence. 

Caroline.  This  weight  of  the  atmosphere,  tlien,  which  I  was 
so  apprehensive  would  crush  me,  is,  in  reality,  essential  to  my 
preservation. 

Emily.  I  once  saw  a  person  cupped,  and  was  told  that  thft 
swelling  of  the  part  under  the  cup,  was  produced  by  taking  away 
from  that  part,  the  pressure  of  the  atmosphere;  but  I  could  not 
understand  how  this  pressure  produced  such  an  effect. 

Mrs.  B.  The  air  pump  aftbrds  us  the  means  of  making  a  great 
variety  of  interesting  experiments,  on  the  weight,  and  pressure  of 

5.  What  is  said  respecting  the  weight  of  the  atmosphere  ?  6.  Why  do  we 
not  feel  the  pressure  of  the  air  ?  7  What  would  be  the  eflfect  of  relieving  us 
from  atmospheric  pressure  ? 

M  ^ 


138  MECHANICAL    PROPERTIES    OF   AIR. 

the  air:  some  of  them  you  have  already  seen.  Do  you  not  recol- 
lect, that  in  a  vacuum  produced  within  the  air  pump,  substances 
of  various  v^^eights,  fell  to  the  bottom  in  the  same  time^  why  does 
not  this  happen  in  the  atmosphere  ? 

Caroline.  I  remember  you  told  us  it  was  owing  to  the  resist- 
ance which  light  bodies  meet  with,  from  the  air,  during  their  fall. 

Mrs.  B.  Or,  in  other  words,  to  the  support  which  they  re- 
ceived from  the  air,  and  which  prolonged  the  time  of  their  fall. 
Now,  if  the  air  were  destitute  of  weight,  how  could  it  support 
other  bodies,  or  retard  their  fall  ? 

I  shall  now  show  you  some  other  experiments,  which  illustrate, 
in  a  striking  manner,  both  tlie  weight,  and  elasticity  of  air.  I 
shall  tie  a  piece  of  bladder  over  this  glass  receiver,  which,  you 
will  observe,  is  open  at  the  top  as  well  as  below. 

Caroline.     Why  do  you  wet  the  bladder  lirst.^ 

Mrs.  B.  It  expands  by  wetting,  and  contracts  in  drying;  it 
is  also  more  soft  and  pliable  when  wet,  so  that  I  can  make  it 
fit  better,  and  when  dry,  it  will  be  tighter.  We  must  hold  it  to 
the  fire  in  order  to  dry  it;  but  not  too  near,  lest  it  should  burst  by 
sudden  contraction.  Let  us  now  fix  it  on  the  air  pump,  and  ex- 
haust the  air  from  underneath  it — ^you  will  not  be  alarmed  if  you 
hear  a  noise  ? 

Emily.  It  was  as  loud  as  the  report  of  a  gun,  and  the  blad- 
der is  burst  I  Pray  explain  how  the  air  is  concerned  in  this  ex- 
periment. 

Mrs.  B.  It  is  the  eftect  of  the  weight  of  the  atmosphere,  on 
the  upper  surface  of  the  bladder,  when  I  had  taken  away  the  air 
from  the  under  surface,  so  that  there  was  no  longer  any  reaction 
to  counterbalance  the  pressure  of  the  atmosphere,  on  the  receiver. 
You  observed  how  the  bladder  was  pressed  inwards,  by  the  weight 
of  the  external  air,  in  proportion  as  I  exhausted  the  receiver; 
and  before  a  complete  vacuum  was  formed,  the  bladder,  unable 
to  sustain  the  violence  of  the  pressure,  burst  with  the  explosion 
you  have  just  heard. 

I  shall  now  show  you  an  experiment,  which  proves  the  expan- 
sion of  the  air,  contained  within  a  body,  when  it  is  relieved  irora 
the  pressure  of  the  external  air.  You  would  not  imagine  that 
there  was  any  air  contained  within  this  shrivelled  apple,  by  its 
appearance;  but  take  notice  of  it  when  placed  within  a  receiver, 
from  which  I  shall  exhaust  the  air. 

Caroline.  How  strange !  it  grows  quite  plump,  and  looks  like 
a  fresh -gathered  apple. 

Mrs.  B.     But  as  soon  as  I  let  the  air  again  into  the  receiver, 

8.  How  may  the  weight  of  the  air  be  shown  by  the  aid  of  the  air  pump, 
and  a  piece  of  bladder  ?  9.  How  is  this  explained  ?  10.  How  jnay  its  eias- 
ticfty  be  exhibited,  by  an  apple,  <4nd  by  a  bladcter? 


MECHANICAL  PROPERTl£S  O*  AIR.  159 

the  apple,  you  see,  returns  to  its  shrivelled  state.  When  I  took 
away  the  pressure  of  the  atmosphere,  the  air  within  the  apple,  ex- 
panded, and  swelled  it  out;  but  the  instant  the  atmospherical  air 
was  restored,  the  expansion  of  the  internal  air,  was  checked  and 
repressed,  and  the  apple  shrunk  to  its  former  dimensions. 

You  may  make  a  similar  experiment  with  this  little  bladder, 
which  you  see  is  perfectly  flaccid,  and  appears  to  contain  no  air: 
in  this  state  I  shall  tie  up  the  neck  of  the  bladder,  so  that  what- 
ever air  remains  within  it,  may  not  escape,  and  then  place  it  un- 
der the  receiver.  Now  observe,  as  I  exhaust  the  receiver,  how 
the  bladder  distends;  this  proceeds  from  the  great  dilatation  of 
the  small  quantity  of  air,  which  was  enclosed  within  the  bladder, 
when  I  tied  it  up;  but  as  soon  as  I  let  the  air  into  the  receiver, 
that  which  the  bladder  contains,  -condenses  and  shrinks  into  its 
small  compass,  within  the  folds  of  the  bladder. 

Emily.  These  experiments  are  extremely  amusing,  and  they 
afford  clear  proofs,  both  of  the  weight,  and  elasticity  of  the  airj 
but  I  should  like  to  know,  exactly,  how  much  the  air  weighs. 

Mrs,  B.  A  column  of  air  reaching  to  the  top  of  the  atmo- 
sphere, and  whose  base  is  a  square  inch,  weighs  about  15lbs. 
therefore,  every  square  inch  of  our  bodies,  sustains  a  weight  of 
lolbs. :  and  if  you  wish  to  know  the  weight  of  the  whole  of  the 
atmosphere,  you  must  reckon  how  many  square  inches  there  are 
on  the  surface  of  the  globe,  and  multiply  them  by  15. 

Emily.  But  can  we  not  ascertain  tne  weight  of  a  small  quan- 
tity of  air? 

Mrs.  B,  With  perfect  ease.  I  shall  exhaust  the  air  from 
this  little  bottle,  by  means  of  the  air  pump:  and  having  emptied 
tlie  bottle  of  air,  or,  in  other  words,  produced  a  vacuum  within 
it,  I  secure  it  by  turning  this  screw  adapted  to  its  neck:  we 
may  now  find  the  exact  weight  of  this  bottle,  by  putting  it  into 
one  of  the  scales  of  a  balance.  It  weighs,  you  see,  just  two 
ounces;  but  when  I  turn  the  screw,  so  as  to  admit  the  air  into 
the  bottle,  the  scale  which  contains  it,  preponderates. 

Caroline.  No  doubt  the  bottle  filled  with  air,  is  heavier  than 
the  bottle  void  of  air;  and  the  additional  weight  required  to 
bring  the  scales  a^ain  to  a  balance,  must  be  exactly  that  of  the 
air  which  the  bottle  now  contains. 

Mrs,  B.  That  weight,  you  see,  is  almost  two  grains.  The 
dimensions  of  this  bottle,  are  six  cubic  inches.  Six  cubic  inches 
of  air,  therefore,  at  the  temperature  of  this  room,  weighs  nearly 
2  grains. 

1 1.  What  is  the  absolute  weight  of  a  given  column  of  atmospheric  air,  and 
how  could  its  whole  pressure  upon  the  earth  be  ascertained  ?  12.  How  can 
the  weight  of  a  small  bulk  of  air  be  found  f 


140  MECHANICAL    PROPERTIES    OF    AIR. 

Caroline.  Why  do  you  observe  the  temperature  of  the  rooiU;^ 
in  estimating  the  weight  of  the  air? 

Mrs.  B.  Because  heat  rarities  air,  and  renders  it  ligliter; 
tlierefore  the  warmer  the  air  is,  which  you  weigh,  the  lighter  it 
will  be. 

If  you  should  now  be  desirous  of  knowing  the  specific  gravity 
of  this  air,  we  need  only  fill  the  same  bottle,  with  water,  and 
thus  obtain  the  weight  of  an  equal  quantity  of  water — which  you 
see  is  1515  grs.;  now  by  comparing  the  weight  of  water,  to  that 
of  air,  we  find  it  to  be  in  the  proportion  of  about  800  to  1. 

As  you  are  acquainted  with  decimal  arithmetic,  you  will  un- 
derstand what  I  mean,  when  I  tell  you,  that  water  being  called 
1000,  the  specific  gravity  of  air,  will  be  1.2. 

I  will  show  you  another  instance,  of  the  weight  of  the  atmo- 
sphere, wliich  I  think  will  please  you:  you  know  what  a  barome- 
ter is.^* 

Caroline.  It  is  an  instrument  which  indicates  the  state  of 
the  weather,  by  means  of  a  tube  of  quicksilver;  but  how,  I  can- 
not exactly  say. 

Mrs.  B.  It  is  by  showing  the  weight  of  the  atmosphere, 
which  has  great  influence  on  the  weather.  The  barometer,  is  an 
instrument  extremely  simple  in  its  construction.  In  order  that 
you  may  understand  it,  I  will  show  you  how  it  is  made.  I  first 
fill  with  mercury,  a  glass  tube  A  fe,  (fig.  3,  plate  14.)  about 
three  feet  in  length,  and  open  only  at  one  end;  then  stopping 
the  open  end,  with  my  finger,  I  immerse  it  in  a  cup  C,  contain- 
ing a  little  mercury. 

Emily.  I^rt  of  the  mercury  which  was  in  the  tube,  I  ob- 
serve, runs  down  into  the  cup;  but  why  does  not  the  whole  of  it 
subside,  for  it  is  contrary  to  the  law  of  the  equilibrium  of  fluids, 
that  the  mercury  in  the  tube,  should  not  descend  to  a  level  with 
tliat  in  the  cup? 

Mrs.  B.  The  mercury  that  has  fallen  from  the  tube,  into  the 
cuj),  has  left  a  vacant  space  in  the  upper  part  of  the  tube,  to 
which  the  air  cannot  gain  access;  this  space  is  therefore  a  per- 
fect vacuum;  the  mercury  in  the  tube,  is  relieved  from  the  pres- 
sure of  the  atmosphere,  whilst  that  in  the  cup,  remains  exposed 
to  it. 

Caroline.  Oh,  now  I  understand  it;  the  pressure  of  the  air 
en  the  mercury  in  the  cup,  forces  it  to  rise  in  the  tube,  where 
there  is  not  any  air  to  counteract  the  external  pressure. 

13.  In  ascertaining  the  weight  of  air,  we  take  account  of  its  temperature  — 
Why  ?  14.  How  could  you  ascertain  the  specific  gravity  of  air,  and  wha*. 
would  it  be  ?  15.  What  are  the  essential  parts  of  a  barometer,  as  representaJ 
plate  14,%,  3? 


R  MECHANICAL    PROPERTIES    OF   AIR,  141 

^mily.     Or  rather  supports  the  mercury  in  the  tube,  and 
vents  it  from  falling. 
Mrs.  B.     That  comes  to  the  same  thing;  for  the  power  that 
I   can  support  mercury  in  a  vacuum,  would  also  make  it  ascend, 
when  it  met  with  a  vacuum. 

Thus  you  see,  that  the  equilibrium  of  the  mercury  is  destroy- 
ed, only  to  preserve  the  general  equilibrium  of  fluids. 

Caroline.  But  this  simple  apparatus  is,  in  appearance,  very 
unlike  a  barometer. 

Mrs.  B.  It  is  all  that  is  essential  to  a  barometer.  The  tube 
and  the  cup,  or  a  cistern  of  mercury,  are  fixed  on  a  board,  for 
the  convenience  of  suspending  it;  the  brass  plate  on  the  upper 
part  of  the  board,  is  graduated  into  inches,  and  tenths  of  incnes, 
for  the  purpose  of  ascertaining  the  height  at  which  the  mer^^^iry 
stands  in  the  tube;  and  the  small  moveable  metal  plate,  serves 
to  show  that  height,  with  greater  accuracy. 

Emily.  And  at  what  height,  will  the  weight  of  the  atmo- 
sphere sustain  the  mercury? 

Mrs.  B.  About  28  or  29  inches,  as  you  will  see  by  this 
barometer;  but  it  depends  upon  the  weight  of  the  atmosphere, 
which  varies  much,  in  different  states  of  the  weather.  The  great- 
er the  pressure  of  the  air  on  the  mercury  in  the  cup,  the  higher 
it  vn[\  ascend  in  the  tube.  Now  can  you  tell  me  whether  the 
air  is  heavier,  in  wet,  or  in  dry  weather? 

Caroline.     Without  a  moment's  reflection,  the  air  must  be 
heaviest  in  wet  weather.     It  is  so  depressing,  and  makes  one 
feel  so  heavy,  while  in  fine  weather,  I  feel  as  light  as  a  feather, 
.  and  as  brisk  as  a  bee. 

Mrs,  B.  Would  it  not  have  been  better  to  have  answered 
with  a  moment's  reflection,  Caroline?  It  would  have  convinced 
you,  that  the  air  must  be  heaviest  in  dry  weather;  for  it  is  then, 
that  the  mercury  is  found  to  rise  in  the  tube,  and  consequently, 
the  mercury  in  the  cup,  must  be  most  pressed  by  the  air. 

Caroline.     Why  then  does  the  air  feel  so  heavy,  in  bad  weather? 

Mrs.  B.  Because  it  is  less  salubrious,  when  impregnated 
with  damp.  The  lungs,  under  these  circumstances,  do  not  play 
so  freely,  nor  does  the  blood  circulate  so  well;  thus  obstructions 
are  frequently  occasioned  in  the  smaller  vessels,  from  which 
arise  colds,  asthmas,  agues,  fevers,  &c. 

Emily.  Since  the  atmosphere  diminishes  in  density,  in  the 
upper  regions,  is  not  the  air  more  rare,  upon  a  hill,  than  in  a 
plain;  and  does  the  barometer  indicate  this  difference? 

16.  What  sustains  the  mercury  in  the  tube?  17.  Of  what  use  are  the 
divisions  in  the  upper  part  of  the  instrument?  18.  To  what  height  will  the 
mercury  rise,  and  what  occasions  this  height  to  vary?  19.  VVhen  is  the 
mercury  highest,  in  wet,  or  in  dry  weather  ?  20.  What  occasions  the  sensa- 
tion of  oppression,  in  damp  weather  ? 


14£  MECHANICAL  PROPERTIES  OF  AIR. 

Mrs.  B.  Certainly.  This  instrument,  is  so  exact  in  its  in- 
dications, that  it  is  used  for  the  purpose  of  measuring  the  height 
of  mountains,  and  of  estimating  the  elevation  of  balloons;  the 
mercury  descending  in  the  tube,  as  you  ascend  to  a  greater 
height. 

Jtlmily.  And  is  no  inconvenience  experienced,  from  the  thin- 
ness of  the  air,  in  such  elevated  situations? 

Mrs.  B.  Oh,  yes;  frequently.  It  is  sometimes  oppressive, 
from  being  insufficient  for  respiration;  and  the  expansion  which 
takes  place,  in  the  more  dense  air  contained  within  the  body,  is 
often  painful :  it  occasions  distention,  and  sometimes  causes  the 
bursting  of  the  smaller  blood-vessels,  in  the  nose,  and  ears.  Be- 
sides in  such  situations,  you  are  more  exposed,  both  to  heat,  and 
cold;  for  though  the  atmosphere  is  itself  transparent,  its  lower 
regions,  abound  with  vapours,  and  exhalations,  from  the  earth, 
which  float  in  it,  and  act  in  some  deoree  as  a  covering,  which 
preserves  us  equally  from  the  intensity  of  the  sun's  rays,  and 
from  the  severity  of  the  cold. 

Caroline.  Pray,  Mrs.  B.,  is  not  the  thermometer  constructed 
on  the  same  principles  as  the  barometer? 

Mrs.  B.  Not  at  all.  The  rise  and  fall  of  the  fluid  in  the 
thermometer,  is  occasioned  by  the  expansive  power  of  heat,  and 
the  condensation  produced  by  cold:  the  air  has  no  access  to  it. 
An  explanation  of  it  would,  therefore,  be  irrelevant  to  our  pre- 
sent subject. 

Emily.  I  have  been  reflecting,  that  since  it  is  the  weight 
of  the  atmosphere,  which  supports  the  meixury,  in  the  tube  of  a 
barometer,  it  would  support  a  column  of  any  other  fluid,  in  the 
same  manner. 

Mrs.  B.  Certainly;  but  as  mercury,  is  heavier  than  all  other 
fluids,  it  will  support  a  higher  column,  of  any  other  fluid;  for  two 
fluids  are  in  equilibrium,  when  their  height  varies,  inversely  as 
their  densities.  We  find  the  weight  of  the  atmosphere,  is  equal 
to  sustaining  a  column  of  water,  for  instance,  of  no  less  than  32 
feet  above  its  level. 

Caroline.  The  weight  of  the  atmosphere,  is  then,  as  great  as 
that  of  a  body  of  water  of  32  feet  in  height. 

Mrs.  B.  Precisely;  for  a  column  of  air,  of  the  height  of  the 
atmosphere,  is  equal  to  a  column  of  water  of  about  '32  feet,  or 
one  of  mercury,  of  from  28  to  29  inches. 

The  common  pump,  is  dependent  on  this  principle.     By  the 

21.  Why  will  the  barometer  indicate  the  height  of  mountains,  or  of  bal- 
loons ?  22.  Is  any  inconvenience  experienced  by  persons  ascending  to  great 
heights,  and  from  what  cause  ?  23.  Wliat  occasions  the  rise  and  fall  of  the 
mercury,  in  a  thermometer  ?  24.  To  what  height  will  the  pressure  of  the 
atmosphere  raise  a  column  of  water  ?  25.  What  governs  the  difference  be- 
tween the  height  of  the  mercury,  and  of  the  water  ? 


MECHANICAL  PROPERTIES  OF  ALR.  143 

act  of  pumpin»,  the  pressure  of  the  atmosphere  is  taken  off  the 
water,  which,  in  consequence,  rises. 

The  body  of  a  pump,  consists  of  a  large  tube  or  pipe,  whose 
lower  end  is  immersed  in  the  water  which  it  is  designed  to  raise. 
A  kind  of  stopper,  called  a  piston,  is  fitted  to  this  tube,  and  is 
made  to  slide  up  and  down  it,  by  means  of  a  metallic  rod,  fastened 
to  the  centre  of  the  piston. 

Emily.  Is  it  not  similar  to  the  syringe,  or  squirt,  with  which 
you  first  draw  in,  and  then  force  out  water? 

Mrs.  B.  It  is5  but  you  know  that  we  do  not  wish  to  force 
the  water  out  of  the  pump,  at  the  same  end  of  the  pipe,  at  which 
we  draw  it  in.  The  intention  of  a  pump,  is  to  raise  water  from 
a  spring,  or  well;  the  pipe  is,  therefore,  placed  perpendicularly 
over  the  water,  which  enters  it  at  the  lower  extremity,  and  it 
issues  at  ^horizontal  spout,  towards  the  upper  part  of  the  pump; 
to  effect  this,  there  are,  besides  the  piston,  two  contrivances 
called  valves.  The  pump,  tlierefore,  is  rather  a  more  compli- 
cated piece  of  machinery,  than  the  syringe. 

Caroline.  Pray,  Mrs.  B. ,  is  not  the  leather,  which  covers  the 
opening,  in  the  lower  board  of  a  pair  of  bellows,  a  kind  of  valve? 

Mrs.  B.  It  is,  valves  are  made  in  various  forms;  any  con- 
trivance, which  allows  a  fluid  to  pass  in  one  direction,  and  pre- 
vents its  return,  is  called  a  valve;  that  of  the  bellows,  and  of  the 
common  pump,  resemble  each  other,  exactly.  You  can  now,  I 
think,  understand  the  structure  of  the  pump. 

Its  various  parts,  are  delineated  in  this  figure:  (fig.  4.  plate 
14.)  A  B  is  the  pipe,  or  body  of  the  pump,  P  the  piston,  V  a 
valve,  or  little  door  in  the  piston,  which,  opening  upwards, 
admits  the  water  to  rise  through  it,  but  prevents  its  returning, 
and  Y,  is  a  similar  valve,  placed  lower  down  in  the  body  of  the 
pump;  H  is  the  handle,  wnich  in  this  model,  serves  to  work  the 
piston. 

When  the  pump  is  in  a  state  of  inaction,  the  two  valves  are 
closed  by  their  own  weight;  but  when,  by  working  the  handle 
of  the  pump,  the  piston  ascends;  it  raises  a  column  of  air  which 
rested  upon  it,  and  produces  a  vacuum,  between  the  piston,  and 
the  lower  valve  Y;  the  au'  beneath  this  valve,  which  is  immedi- 
ately over  the  surface  of  the  water,  consequently  expands,  and 
forces  its  way  through  it;  the  water,  then,  relieved  from  the 
pressure  of  the  air,  ascends  into  the  pump.  A  few  strokes  of 
the  handle,  totally  excludes  the  afr  from  the  body  of  the  pump, 
and  fills  it  witli  water,  which,  having  passed  through  both  the 
valves,  runs  out  at  the  spout. 

Caroline.     I  understand  this  perfectly.     When  the  piston  is 

26.  How  does  the  common  pump,  raise  water  from  a  well  ?  27.  What 
is  meant  by  a  piston  ?  28.  Describe  the  construction,  and  use,  of  a  valve. 
29.  What  are  the  parts  of  the  pump,  as  represented,  %.  4,  plate  14  ? 


144  MECHANICAL  PROPERTIES  OF  AIR. 

elevated,  the  air,  and  the  water,  successively  rise  in  the  pump, 
for  the  same  reason  as  the  mercury,  rises  in  the  barometer. 

Emily,  I  thought  that  water  was  drawn  up  into  a  pump,  by 
suction,  in  the  same  manner  as  water  may  be  sucked  through  a 
•traw. 

Mrs,  B,  It  is  so,  into  the  body  of  the  pump;  for  the  power 
of  suction,  is  no  other  than  that  of  producing  a  vacuum  over  one 
part  of  the  liquid,  into  which  vacuum  the  liquid  is  forced,  by 
the  pressure  of  the  atmosphere,  on  another  part.  The  action  of 
sucking  through  a  straw,  consists Jn  drawmg  in,  and  confining 
the  breath,  so  as  to  produce  a  vacuum  in  the  mouth;  in  conse- 
quence of  which,  the  air  within  the  straw,  nishes  into  the  mouth, 
and  is  followed  by  the  liquid,  into  which,  the  lower  end  of  the 
straw,  is  immersed.  The  principle,  you  see,  is  the  same,  and 
the  only  difference  consists  in  the  mode  of  producing  a  vacuum. 
In  suction,  the  muscular  powers  answer  the  purpose  of  the  pis- 
ton and  valve. 

Emily,  Water  cannot,  then,  be  raised  by  a  pump,  above  32 
feet;  for  the  pressure  of  the  atmosphere  will  not  sustain  a  column 
of  water,  above  that  height. 

Mrs.  B,  I  beg  your  pardon.  It  is  true  that  there  must 
never  be  so  great  a  distance  as  32  feet,  from  the  level  of  the 
water  in  the  well,  to  the  valve  in  the  piston,  otherwise  the 
water  would  not  rise  through  that  valve;  but  when  once  the 
water  has  passed  that  opening,  it  is  no  longer  the  pressure  of 
air  on  the  reservoir,  which  makes  it  ascend;  it  is  raised  by  lift- 
ing it  up,  as  you  would  raise  it  in  a  bucket,  of  which  the  piston 
formed  the  bottom.  This  common  pump  is,  therefore,  called 
the  sucking,  or  liftinff  pump,  as  it  is  constructed  on  both  these 
principles.  The  rod  to  which  the  piston  is  attached,  must  be 
made  sufficiently  long,  to  allow  the  piston  to  be  within  32  feet 
of  the  surface  oi  the  water  in  the  well,  however  deep  it  may  be. 
There  is  another  sort  of  pump,  called  the  forcing  pump:  it  con- 
sists of  a  forcing  power,  added  to  the  sucking  part  of  the  pump. 
This  additional  power,  is  exactly  on  the  principle  of  the  syringe: 
by  raising  the  piston,  you  draw  the  water  into  the  pump,  and  by 
causing  it  to  descend,  you  force  the  water  out. 

Caroline,  But  the  water  must  be  forced  out  at  the  upper 
part  of  the  pump;  and  I  cannot  conceive  how  that  can  be  done 
hy  the  descent  of  the  piston. 

Mrs,  B,  Figure  5,  plate  14,  will  explain  the  difficulty.  The 
large  pipe,  A  B,  represents  the  sucking  part  of  the  pump,  wifiich 
differs  from  the  lifting  pump,  only  in  its  piston  P,  being  unfurn- 

30.  How  do  these  parts  act,  in  raising  the  water  ?  31.  In  what  doea  that 
whichis  commonly  called  auction,  consist  ?  32,  How  must  the  piston  be  litu- 
ated  in  the  pump  ?    33.  What  other  kind  of  pump  is  described ' 


MECHANICAL  PROPERTIES  OF  AIR.  145 

ished  with  a  valve,  in  consequence  of  which  the  water  cannot 
rise  above  it.  When,  therefore,  the  piston  descends,  it  shuts 
the  valve  Y,  and  forces  the  water  (which  has  no  other  vent)  into 
the  pipe  D:  this  is  likewise  furnished  with  a  valve  V,  which, 
opening  upwards,  admits  the  water  to  pass,  but  prevents  its 
return. 

The  water,  is  thus  first  raised  in  the  pump,  and  then  forced 
into  the  pipe,  by  the  alternate  ascending,  and  descending  motion 
of  the  piston,  after  a  few  strokes  of  the  handle  to  fill  the  pipe, 
from  whence  the  water  issues  at  the  spout. 

Emily.  Does  not  the  air  pump,  which  you  used  in  the  experi- 
ments, oti  pneumatics,  operate  upon  the  same  principles  as  the 
sucking  piimp? 

Mrs.  JB.  Exactly.  The  air  pumpwhicli  I  used  (plate  1,  fig. 
2,)  has  two  hollow,  brass  cylinders,  called  barrels,  which  are 
made  perfectly  true.  In  each  of  those  barrels,  there  is  a  piston^ 
these  are  worked  up,  and  down,  by  the  same  handle;  the  pistons, 
are  furnished  with  valves,  opening  upwards,  like  those  of  the 
common  pump:  there  are  valves  also,  placed  at  the  lower  part 
of  each  barrel,  wliich  open  upwards;  there  are  therefore  two 
pumps,  united  to  produce  the  same  effect:  two  tubes,  connect 
these  barrels  with  the  plate,  upon  which  I  placed  the  receivers, 
which  were  to  be  exhausted. 

Emily.  I  now  understand  how  the  air  pump  acts;  the  re- 
ceiver contains  air,  which  is  exhausted,  just  as  it  is  by  the  com- 
mon pump,  before  the  water  begins  to  rise. 

Mrs.  B.  Having  explained  the  mechanical  properties  of  air, 
I  think  it  is  now  time  to  conclude  our  lesson.  When  next  we 
meet,  I  shall  give  you  some  account  of  \vind,  and  of  sound,  which 
will  terminate  our  observations  on  elastic  fluids. 

Caroline.  And  I  shall  run  into  the  garden,  to  have  the  plea 
sure  of  pumping,  now  that  I  understand  the  construction  of  a 
pump. 

Mrs.  B.  And,  to-morrow,  I  liope  you  will  be  able  to  tell  me, 
whether  it  is  a  forcing,  or  a  common  lifting  pump. 

34.  How  is  the  forcing  pump  constructed,  as  shown  in  plate  14,  fig.  5  ?  35. 
Describe  the  conBtruction  and  operation  of  the  air  pump,  (fig.  2,  plat©  !•) 


N 


CONVERSATION  XIH. 


ON  WIND  AND  SOUND- 

9V    WIND    IN    GENERAL. — OF    THE    TRADE-WIND. — OF    THE    PERIODICAL 
TRADE-WINDS. — OF  THE  AERIAL  TIDES. — OF  SOUNDS  IN  GENERAL. — OF 

SONOROUS  BODIES. — OF  MUSICAL  SOUNDS. OF   CONCORD  OR  HARMONY, 

AND  MELODY. 

MRS.    B. 

Well,  Caroline,  have  you  ascertained  what  kind  of  pump  you 
have  in  your  garden? 

Caroline.  I  think  it  must  be  merely  a  lifting  pump,  because 
no  more  force  is  required  to  raise  the  handle  than  is  necessary 
to  lift  its  vi^eight  J  and  as  in  a  forcing  pump,  by  raising  the  handle, 
you  force  the  water  into  the  smaller  pipe,  the  resistance  the 
water  offers,  must  require  an  exertion  of  strength,  to  overcome  it. 
,  Mrs.  B.  I  make  no  doubt  you  are  rights  for  lifting  pumps, 
being  simple  in  their  construction,  are  by  far  the  most  common. 

I  have  promised  to  day  to  give  you  some  account  of  the  nature 
of  wind.  Wind  is  nothing  more  than  the  motion  of  a  stream,  or 
current  of  air,  generally  produced  by  a  partial  change  of  temper- 
ature in  the  atmosphere;  for  when  any  one  part  is  more  heated 
than  the  rest,  that  part  is  rarefied,  the  air  in  consequence  rises, 
and  the  equilibrium  is  destroyed.  W^hen  this  happens,  there 
necessarily  follows  a  motion  ot  the  surrounding  air  towards  that 
part,  in  order  to  restore  it;  this  spot,  therefore,  receives  winds 
from  every  quarter.  Those  who  live  to  the  north  of  it,  experi- 
ence a  north  wind;  those  to  the  south,  a  south  wind: — do  you 
comprehend  this? 

Caroline.  Perfectly.  But  what  sort  of  weather  must  those 
people  have,  who  live  on  the  spot,  where  these  winds  meet  and 
mterfere? 

Mrs.  B.  They  have  most  commonly  turbulent  and  boister- 
ous weather,  whirlwinds,  hurricanes,  rain,  lightning,  thunder, 
&c.  This  stormy  weather  occurs  most  frequently  in  the  torrid 
zone,  where  the  heat  is  greatest:  the  air  being  more  rarefied 

1,  What  is  wind,  and  how  is  it  generally  produced  ?  2.  How  do  the  winds 
blow,  around  the  place  where  the  air  becomes  rarefied  ?  3.  What  effect  is 
likely  to  be  produced  where  the  ^vinds  meet  .•• 


ON   WIND    ANDT  SOUND.  147 

there,  than  in  any  other  part  of  the  globe,  is  lighter,  and  conse- 
quently, ascends;  whilst  the  air  from  the  north  and  south,  is 
continually  flowing  in,  to  restore  the  equilibrium. 

Caroline.  This  motion  of  the  air,  would  produce  a  regular 
and  constant  north  wind,  to  the  inhabitants  of  the  northern 
hemisphere;  and  a  south  wind,  to  those  of  the  southern  hemi- 
sphere, and  continual  storms  at  the  equator,  where  these  two  ad- 
verse winds  would  meet. 

Mrs.  B.  These  winds  do  not  meet,  for  they  each  change 
their  direction  before  they  reach  the  equator.  The  sun,  in  mov- 
ing over  the  equatorial  regions  from  east  to  west,  rarefies  the 
air  as  it  passes,  and  causes  the  denser  eastern  air  to  flow  west- 
wards, in  order  to  restore  the  equilibrium,  thus  producing  a  re- 
gular east  wind,  about  the  equator. 

Caroline.  The  air  from  the  west,  then,  constantly  goes  to 
meet  the  sun,  and  repair  the  disturbance  which  his  beams  have 
produced  in  the  equilibrium  of  the  atmosphere.  But  I  wonder 
now  you  will  reconcile  these  various  winds,  Mrs.  B.;  you  first 
led  me  to  suppose  there  was  a  constant  struggle  between  oppo- 
site winds  at  the  equator,  producing  storm  and  tempest;  but 
now  I  hear  of  one  regular  invariable  wind,  which  must  naturally 
be  attended  by  calm  weather. 

Emily,  I  think  I  comprehend  it:  do  not  these  winds  from 
the  north  aiid  south,  combine  with  the  easterly  v/ind  a^out  the 
equator,  and  form,  what  are  called,  the  trade- winds? 

3Ir8.  B.  Just  so,  my  dear.  The  composition  of  the  two 
winds,  north  and  east,  produces  a  constant  north-east  wind;  and 
that  of  the  two  winds,  south  and  east,  produces  a  regular  south- 
east wind;  these  winds  extend  to  about  thirty  degrees  on  each 
side  of  the  equator,  the  regions  further  distant  from  it,  expe- 
riencing only  their  respective  northerly  and  southerly  winds 

Caroline.     But,  Mrs.  B.,  if  the  air  is  constantly  flowing  fj 
the  poles,  to  the  torrid  zone,  there  must  be  a  deficiency  of  air, 
in  the  polar  regions? 

Mrs.  B.^  The  light  air  about  the  equator,  which  expands, 
and  rises  into  the  upper  regions  of  the  atmosphere,  ultimately 
flows  from  thence,  bacK  to  the  poles,  to  restore  the  equilibrium; 
if  it  were  not  for  this  resource,  the  polar,  atmospheric  regions, 
would  soon  be  exhausted  by  the  stream  of  air,  which,  in  the 
lower  strata  of  the  atmosphere,  they  are  constantly  sending  to- 
wards the  equator. 

Caroline.     There  is  then  a  sort  of  circulation  of  air  in  the  at- 

4.  In  what  part  of  the  globe  is  the  air  most  rarefied,  and  what  is  the  con- 
sequence ?  5.  How  do  these  winds  change  their  direction  as  they  approach 
the  equator  ?  6.  How  are  the  trade-winds  produced,  and  how  far  do  they 
expend  ?    7.  How  is  the  equilibrium  in  the  air  restored 


J,  from 


t48  OJf    WIND    AND    SOUND. 

mosphere;  the  air  in  the  lower  strata,  flowing  from  the  poles  to- 
wards the  equator,  and  in  the  upper  strata,  flowing  back  from 
the  equator,  towards  the  poles. 

Mrs.  B.  Exactly 5  I  can  show  you  an  example  of  this  circu- 
lation, on  a  smaller  scale.  The  air  of  this  room,  being  more 
rarefied,  than  the  external  air,  a  wind  or  current  of  air  is  pour- 
ing in  from  the  crevices  of  the  windows  and  doors,  to  restore  the 
equilibrium  j  but  the  light  air,  with  which  the  room  is  filled,  must 
find  some  vent,  in  order  to  make  way  for  the  heavy  air  that  en- 
ters, ''if  you  set  the  door  a-jar,  and  hold  a  candle  near  the  up- 
per part  of  it,  you  will  find  that  the  flame  will  be  blown  out- 
wards, showing  that  there  is  a  current  of  air  flowing  out  from  the 
upper  part  of  the  room. — Now  place  the  candle  on  the  floor, 
close  by  the  door,  and  you  will  perceive,  by  the  inclination  of 
the  flame,  that  there  is  also  a  current  of  air,  setting  into  the 
room. 

Caroline.  It  is  just  so;  the  upper  current  is  the  warm  light 
air,  which  is  driven  out  to  make  way  for  the  stream  of  cold 
dense  air,  which  enters  the  room  lower  down. 

Mrs.  B.  Besides  the  general,  or  trade-winds,  there  aie 
others,  which  are  called  periodical,  because  they  blow  in  con- 
rary  directions,  at  particular  periods-. 

Emily.  I  have  heard,  Mrs.  B.,  that  the  periodical  winds, 
.ailed,  in  the  torrid  zone,  the  sea  and  land  breezes,  blow  to- 
Avards  the  land,  in  the  day  time,  and  towards  the  sea,  at  night: 
'A'hat  is  the  reason  of  that? 

Mrs.  B.  The  land  reflects  into  the  atmosphere,  a  much 
j^reater  quantity  of  the  sun's  rays,  than  the  water;  therefore, 
liat  part  of  the  atmosphere  which  is  over  the  land,  is  more 
Ilea  ted  and  rarefied,  than  that  which  is  over  the  sea:  this  occa- 
sions the  wind  to  set  in  upon  tiie  land,  as  we  find  that  it  regu- 
larly does  on  the  coast  of  Guinea,  and  other  countries  in  the 
torrid  zone.  There,  they  have  only  the  sea  breeze,  but  on  the 
islands,  they  have,  in  general,  both  a  land  and  sea  breeze,  the 
latter  being  produced  in  the  way  described;  whilst  at  night, 
during  the  absence  of  the  sun,  the  earth  cools,  and  the  air  is 
consequently  condensed,  and  flows  from  the  land,  towards  the 
<ea,  occasioning  the  land  breeze. 

Emily.  I  have  heard  much  of  the  violent  tempests,  occa- 
sioned by  the  breaking  up  of  the  monsoons;  are  not  they  also 
regular  trade -winds. ^ 

Mrs.  B.  They  are  called  periodical  trade-winds,  as  they 
iiange  their  course  every  half  year.     Tliis  variation  is  produced 

8  How  can  contrary  currents  of  air  be  shown  in  a  ro6m  ?     9.  What  causes 
lie?     10.  What  is  meant  by  a  periodical  wind?     11.  What  occasions  thf 
^iid  and  sea  breezes,  and  where  do  they  prevail  ? 


ON    WIND    AND    SOUI^D.  149 

by  the  earth's  annual  course  round  the  sun^  the  north  pole  being 
inclined  towards  that  luminary  one  half  of  the  year,  the  south 
pole,  the  other  half.  During  the  summer  of  the  northern  hemi- 
sphere, the  countries  of  Arabia,  Persia,  India,  and  China,  are 
much  heated,  and  reflect  great  quantities  of  the  sun's  rays  into 
the  atmosphere,  by  which  it  becomes  extremely  rarefied,  and 
the  equilibrium  consequently  destroyed.  In  order  to  restore  it, 
the  air  from  the  equatorial  southern  regions,  where  it  is  colder, 
(as  well  as  from  the  colder  northern  parts,)  must  necessarily 
have  a  motion  towards  those  parts.  The  current  of  air  from  the 
equatorial  regions,  produces  the  trade-winds  for  the  first  six 
months,  in  all  the  seas  between  the  heated  continent  of  Asia, 
and  the  equator.  The  other  six  months,  when  it  is  summer  in 
the  southern  hemisphere,  the  ocean  and  countries  towards  the 
southern  tropic  are  most  heated,  and  the  air  over  those  parts, 
more  rarefied:  then  the  air  about  the  equator  alters  its  course, 
and  flows  exactly  in  an  opposite  direction. 

Caroline.  This  explanation  of  the  monsoons  is  very  curious 5 
but  what  does  their  breaking  up  mean  ? 

Mrs.  B.  It  is  the  name  given  by  sailors  to  the  shifting  of 
the  periodical  winds;  they  do  not  change  their  course  suddenly, 
but  by  degrees,  as  the  sun  moves  from  one  hemisphere,  to  the 
othpr:  this  change  is  usually  attended  by  storms  and  hurricanes^ 
very  dangerous  for  shipping;  so  that  those  seas  are  seldom  navi- 
gated at  the  season  of  the  equinoxes.    " 

Emily.  I  think  I  understand  the  winds  in  the  torrid  zone 
perfectly  well;  but  what  is  it  that  occasions  the  great  variety  of 
winds,  which  occur  in  the  temperate  zones?  for,  according  to 
your  theory,  there  should  be  only  north  and  south  winds,  in 
those  climates. 

Mrs.  B.  ^Since  so  large  a  portion  of  the  atmosphere,  as  is 
over  the  torrid  zone,  is  in  continued  ao-itation,  these  agitations 
in  an  elastic  fluid,  which  yields  to  the  slightest  impression,  must 
extend  every  way,  to  a  great  distance;  the  air,  therefore,  in  all 
climates,  wdl  suffer  more  or  less  perturbation,  according  to  the 
situation  of  the  country,  the  position  of  mountains,  valleys,  and 
a  variety  of  other  causes:  hence  it  is  easy  to  conceive,  that  aU 
most  every  climate,  must  be  liable  to  variable  winds;  this  is  par- 
ticularly the  case  in  high  latitudes,  where  the  earth  is  less  pow- 
erfully affected  by  the  sun's  rays,  than  near  the  equator. 

Caroline.  I  have  observed,  that  the  wind,  whichever  way  it 
blows,  almost  always  falls  about  sun-set. 

12.  What  are  monsoons  ?  13.  How  do  they  change,  and  what  is  the  cause  ? 
14.  What  is  meat^t  by  their  breaking  up,  and  what  effect  is  in  general  pro- 
ctuced.^     15.  Why  is  the  wind  most  variable  in  high  latitudes  ^ 

N2 


)0  ON    WIND    AND    SOUND. 

Mrs,  B,  Because  the  rarefaction  of  air  in  the  particular  spot 
which  produces  the  wind,  diminishes  as  the  sun  declines,  and 
consequently  the  velocity  of  the  wind,  abates. 

Emily.  Since  the  air  is  a  gravitating  fluid,  is  it  not  aft'ected 
by  the  attraction  of  the  moon  and  the  sun,  in  the  same  manner 
IS  the  waters? 

Mrs.  B.  Undoubtedly;  but  the  aerial  tides  are  as  much  great- 
er than  those  of  water,  as  the  density  of  water  exceeds  that  of 
air,  which,  as  you  may  recollect,  we  found  to  be  about  800  to  1. 
Caroline.  What  a  prodigious  protuberance  that  must  occa- 
sion !  How  much  the  weight  of  such  a  column  of  air,  must  raise 
the  mercury  in  the  barometer ! 

Emily.  As  this  enormous  tide  of  air  is  drawn  up  and  sup- 
ported, as  it  were,  by  the  moon,  its  weight  and  pressure,  I 
should  suppose,  would  be  rather  diminished  than  increased? 

Mrs.  B.  The  weight  of  the  atmosphere  is  neither  increased 
nor  diminished  by  the  aerial  tides.  Tlie  moon's  attraction  aug- 
ments the  bulk,  as  much  as  it  diminishes  the  weight,  of  the  co- 
lumn of  air;  these  effects,  therefore,  counterbalancing  each  other, 
the  aerial  tides  do  not  affect  the  barometer. 
Caroline.  I  do  not  quite  understand  that. 
3l7's.  B.  Let  us  suppose  that  the  additional  bulk  of  air  at 
high  tide,  raises  the  barometer  one  inch;  and  on  the  other  hand, 
that  the  support  which  the  moon's  attraction  affords  the  air,  di- 
minishes its  weight  or  pressure,  so  as  to  occasion  the  mercury 
to  fall  one  inch;  under  these  circumstances  the  mercury  must 
remain  stationary.  Thus,  you  see,  that  we  can  never  be  sensi- 
ble of  serial  tides  by  the  barometer,  on  account  of  the  equality 
of  pressure  of  the  atmosphere,  whatever  be  its  height. 

The  existence  of  aeiial  tides  is  not,  however,  hypothetical;  it 
is  proved  by  the  effect  they  produce  on  the  apparent  position  of 
the  heavenly  bodies;  but  this  I  cannot  explain  to  you,  till  you 
understand  the  properties  of  light. 
Emily.  And  when  shall  we  learn  them? 
Mrs.  B.  I  shall  first  explain  to  you  the  nature  of  sound, 
which  is  intimately  connected  with  tliat  of  air;  and  I  think  at 
fur  next  meeting,  we  may  enter  upon  the  subject  of  optics. 

We  have  now  considered  the  eff*ects  produced  by  the  wide, 

and  extended  agitation,  of  the  air;  but  there  is  another  kind  of 

agitation,  of  which  the  air  is  susceptible — a  vibratory  trembling 

motion,  which,  striking  on  the  drum  of  the  ear,  produces  sound. 

Caroline.     Is  not  sound  produced  by  solid  bodies?    The  voice 

16.  Why  is  the  wind  apt  to  lessen  about  sunset?  17.  What  effect  must 
liie  sun  and  mo<m  produce  upon  the  atmosphere,  from  their  attraction? 
28   Why  do  not  the  aerial  tides  affect  the  barometer.'* 


9li    WIND    AND    SOUND.  I5l 

of  animals,  the  ringing  of  bells,  the  music  of  instruments,  all 
proceed  from  solid  bodies.  I  know  of  no  sound  but  that  of  the 
wind,  which  is  produced  by  the  air. 

Mrs.  B.  Sound,  I  assure  you,  results. from  a  tremulous  mo- 
tion of  the  air;,  and  the  sonorous  bodies  you  enumerate,  are 
merely  the  instruments  hy  which  that  peculiar  species  of  motion, 
is  communicated  to  the  air. 

Caroline.  What !  when  I  ring  this  little  bell,  is  it  the  air  that 
sounds,  and  not  the  bell.'* 

Mrs.  B.  Botli  the  bell,  and  the  air,  are  concerned  in  the  pro- 
duction of  sound.  But  sound,  strictly  speaking,  is  a  perception 
excited  in  the  mind,  by  the  motion  of  the  air,  on  the  nerves  of 
the  ear;  the  air,  therefore,  as  well  as  the  sonorous  bodies  which 
put  it  in  motion,  is  only  the  cause  of  sound,  the  immediate  ef- 
fect is  produced  by  the  sense  of  hearing:  for  without  this  sense, 
there  would  be  no  sound. 

Emily.  I  can  with  difficulty  conceive  that.  A  person  born 
deaf,  it  is  true,  has  no  idea  of  sound,  because  he  hears  none;  yet 
that  does  not  prevent  the  real  existence  of  sound,  as  all  those 
who  are  not  deaf,  can  testify. 

Mrs.  B.  I  do  not  doubt  the  existence  of  sound,  to  all  those 
who  possess  the  sense  of  hearing;  but  it  exists  neither  in  the 
sonorous  body,  nor  in  the  air,  but  in  the  mind  of  the  person 
whose  ear  is  struck,  by  the  vibratory  motion  of  the  air,  produced 
by  a  sonorous  body.  Sound,  therefore,  is  a  sensation,  produced 
in  a  living  body;  life,  is  as  necessary  to  its  existence,  as  it  is  to 
that  of  feeling  or  seeing. 

To  convince  you  that  sound  does  not  exist  in  sonorous  bodies, 
but  that  air  or  some  other  vehicle,  is  necessary  to  its  production, 
endeavour  to  ring  the  little  bell,  after  I  have  suspenaed  it  under 
a  receiver  in  the  air  pump,  from  which  I  shall  exhaust  the  air.... 

Caroline.  This  is  indeed  very  strange :  though  I  agitate  it  so 
violently,  it  produces  but.  little  sound. 

Mrs.  B.  By  exhausting  the  receiver,  I  have  cut  off  the  com- 
Kiunication  between  the  air  and  the  bell;  the  latter,  therefore, 
cannot  impart  its  motion,  to  the  air. 

Caroline.  Are  you  sure  that  it  is  not  the  glass,  which  covers 
the  bell,  that  prevents  our  hearing  it? 

Mrs.  B.  That  you  may  easily  ascertain,  by  letting  the  air 
into  the  receiver,  and  then  ringing  the  bell. 

Caroline.  Very  true:  I  can liear  it  now,  almost  as  loud,  as  if 
the  glass  did  not  cover  it;  and  I  can  no  longer  doubt  but  that 
air  is  necessary  to  the  production  of  sound. 

19.  How  is  sound  produced  ?  20.  Does  sound  exist  in  the  sonorous  body, 
if  not,  what  is  it?  21.  By  what  experiment  might  we  prove  that  air  is  the 
principal  vehicle  of  sound  ? 


152  ON    WIND    AND    SOUND. 

Mrs,  B.  Not  absolutely  necessary,  though  b^f  far  the  most 
common  vehicle  of  sound.  Liquids,  as  well  as  air,  are  capable 
of  conveying  the  vibratory  motion  of  a  sonorous  body,  to  the 
organ  of  hearing;  as  sound  can  be  heard  under  water.  Solid 
bodies  also,  convey  sound,  as  I  can  soon  convince  you  by  a  very 
simple  experiment.  I  shall  fasten  this  string  by  the  middle, 
round  the  poker;  now  raise  the  poker  from  the  ground,  by  the 
two  ends  of  the  string,  and  hold  one  to  each  of  your  ears:— -I 
shall  now  strike  the  poker,  with  a  key,  and  you  will  find  that 
the  sound  is  conveyed  to  the  ear  by  means  of  the  strings,  in  a 
much  more  perfect  manner,  than  if  it  had  no  other  vehicle  than 
the  air. 

Caroline.  That  it  is,  certainly,  for  I  am  almost  stunned  by 
the  noise.  But  what  is  a  sonorous  body,  Mrs.  B.  ?  for  all  bo- 
dies are  capable  of  producing  some  kind  of  sound,  by  the  motion 
they  communicate  to  the  air. 

Mrs,  B,  Those  bodies  are  called  sonorous,  which  produce 
clear,  distinct,  re^lar,  and  durable  sounds,  such  as  a  bell,  a 
drum,  musical  strings,  wind  instruments,  &c.  They  owe  this 
property  to  their  elasticity;  for  an  elastic  body,  after  having 
been  struck,  not  only  returns  to  its  former  situation,  but  having 
acquired  momentum  by  its  velocity,  like  the  pendulum,  it 
springs  out  on  the  opposite  side.  If  I  draw  the  string  A  B,  (fig. 
6,  plate  14,)  which  is  made  fast  at  both  ends,  to  C,  it  will  not 
only  return  to  its  original  position,  but  proceed  onwards,  to  D. 

This  is  its  first  vibration;  at  the  end  of  which,  it  will  retain 
sufficient  velocity  to  bring  it  to  E,  and  back  again  to  F,  which 
constitutes  its  second  vibration;  the  third  vibration,  will  carry  it 
©nly  to  G  and  H,  and  so  on,  till  the  resistance  of  the  air  destroys 
its  motion. 

The  vibration  of  a  sonorous  body,  gives  a  tremulous  motion  to 
the  air  around  it,  very  similar  to  the  motion  communicated  to 
smooth  water,  when  a  stone  is  thrown  into  it.  This,  first  pro- 
duces a  small  circular  wave,  around  the  spot  in  which  the  stone 
falls;  the  wave  spreads,  and  gradually  communicates  its  motion 
to  the  adjacent  waters,  producing  similar  waves  to  a  consider- 
able extent.  The  same  kind  of  waves  are  produced  in  the  air,  by 
the  motion  of  a  sonorous  body,  but  with  this  difference,  that  as 
air,  is  an  elastic  fluid,  the  motion  does  not  consist  of  regularly 
extending  waves,  but  of  vibrations;  and  are  composed  of  a  mo- 
tion, forwards  and  backwards,  similar  to  those  of  the  sonorous 

22.  What  other  bodies  convey  sound,  and  how  can  it  be  shown  that  they 
do  so  ?  23.  What  is  meant  by  a  sonorous  body  ?  24.  To  what  do  tliey  owe 
this  property  ?  25.  How  is  this  explained  by  fig.  6,  plate  14  ?  26.  How  is 
it  illustrated  by  a  stone  thrown  into  water,  and  how  far  does  this  illustration 
apply? 


ON    WIND    AND    SOUN».  153 

body.  They  differ  also,  in  the  one  taking  place  in  a  plane,  the 
other,  in  all  directions:  the  aerial  undulations,  being  spherical. 

Emily.  But  if  the  air  moves  backwards,  as  well  as  forwards, 
how  can  its  motion  extend  so  as  to  convey  sound  to  a  distance? 

Mrs.  B.  The  first  sphere  of  undulations,  which  are  produced 
immediately  around  the  sonorous  body,  by  pressing  against  the 
contiguous  air,  condenses  it.     The  condensed  air,  though  ini- 

f)elled  forward  by  the  pressure,  reacts  on  the  first  set  of  undu- 
ations,  driving  them  back  again.  The  second  set  of  undula- 
tions which  have  been  put  in  motion,  in  their  turn,  communicate 
their  motion,  and  are  themselves  driven  back,  by  reaction. 
Thus,  there  is  a  succession  of  waves  in  the  air,  corresponding 
with  the  succession  of  waves  in  the  water. 

Caroline.  The  vibrations  of  sound,  must  extend  much  further 
than  the  circular  waves  in  water,  since  sound  is  conveyed  to  a 
great  distance. 

Mrs.  B.  The  air  is  a  fluid  so  much  less  dense  than  water, 
that  motion  is  more  easily  communicated  to  it.  The  report  of  a 
cannon  produces  vibrations  of  the  air,  which  extend  to  several 
miles  around. 

Emily.  Distant  sound  takes  some  time  to  reach  us,  since  it 
is  produced  at  the  moment  the  cannon  is  fired;  and  we  see  the 
light  of  the  flash,  lon^  before  we  hear  the  report. 

Mrs.  B.  The  air  is  immediately  put  in  motion,  by  the  firing 
of  a  cannon;  but  it  requires  time  for  the  vibrations  to  extend  to 
any  distant  spot.  The  velocity  of  sound,  is  computed  to  be  at 
the  rate  of  1142  feet  in  a  second. 

Caroline.  With  what  astonishing  rapidity  the  vibrations 
must  be  communicated !  But  the  velocity  of  sound  varies,  I  sup- 
pose, with  that  of  the  air  which  conveys  it.  If  the  wind  sets 
towards  us  from  the  cannon,  we  must  hear  the  report  sooner  than 
if  it  set  the  other  way. 

Airs.  B.  The  direction  of  the  wind  makes  less  difference  in 
the  velocity  of  sound,  than  you  would  imagine.  If  the  wind  sets 
from  us,  it  bears  most  of  the  aerial  waves  away,  and  renders  the 
sound  fainter;  but  it  is  not  very  considerably  longer  in  reaching 
the  ear,  than  if  the  wind  blew  towards  us.  This  uniform  velo- 
city of  sound,  enables  us  to  determine  the  distance  of  the  object, 
from  which  it  proceeds;  as  that  of  a  vessel  at  sea,  firing  a  cannon, 
or  that  of  a  thunder  cloud.  If  we  do  not  hear  the  thunder,  till 
half  a  minute  after  we  see  the  lightning,  we  conclude  the  cloud 
to  be  at  the  distance  of  six  miles  and  a  half. 

27.  How  are  the  vibrations  propagated?  28.  How  can  we  prove  that 
sound^does  not  travel  as  rapidly  as  light  ?  29.  At  what  rate  is  sound  said  to 
travel  ?  30.  Is  the  velocity  much  influenced  by  the  direction  of  the  wind  ? 
31.  How  will  sound  enable  us  to  judge  of  the  distance  of  objeeta  ? 


154  ON   WIND    AND    SOUNB. 

Emily.     Pray,  how  is  the  sound  of  an  echo  produced? 

Mrs,  B,  When  the  serial  vibrations  meet  with  an  obstacle, 
having  a  hard  and  regular  surface,  such  as  a  wall,  or  rock,  they 
are  reflected  back  to  the  ear,  and  produce  the  same  sound  a  se- 
cond time;  but  the  sound  will  then  appear  to  proceed,  from  the 
object  by  which  it  is  reflected.  If  trie  vibrations  fall  perpen- 
dicularly on  the  obstacle,  they  are  reflected  back  in  the  same 
line;  if  obliquely,  the  sound  returns  obliquely,  in  the  opposite 
direction,  the  angle  of  reflection  being  equal  to  the  angle  of  in- 
cidence. 

Caroline.  Oh,  then,  Emily,  I  now  understand  why  the  echo 
of  my  voice  behind  our  house  is  heard  so  much  plainer  by  you 
than  it  is  by  me,  when  we  stand  at  the  opposite  ends  of  the 
gravel  walk.  My  voice,  or  rather,  I  should  say,  the  vibrations 
of  air  it  occasions,  fall  obliquely  on  the  wall  of  the  house,  and 
are  reflected  by  it,  to  the  opposite  end  of  the  gravel  walk. 

Emily.  Very  true;  and  we  have  observed,  that  when  we 
stand  in  the  middle  of  the  walk,  opposite  the  house,  the  echo  re- 
turns to  the  person  who  spoke. 

Mrs.  B.  Speaking-trumpets,  are  constructed  on  the  principle, 
that  sound  is  reflected.  The  voice,  instead  of  being  diftused  in 
the  open  air,  is  confined  within  the  trumpet;  and  the  vibrations 
which  would  otherwise  spread  laterally,  fall  against  the  sides  of 
the  instrument,  and  are  reflected  from  the  different  points  of  in- 
cidence, so  as  to  combine  with  those  vibrations  which  proceed 
straight  forwards.  The  vibrations  are  thus  forced  onwards,  in 
the  direction  of  the  trumpet,  so  as  greatly  to  increase  the  sound, 
to  a  person  situated  in  that  direction.  Figure  7,  plate  14,  will 
give  you  a  clearer  idea,  of  the  speaking-trumpet;  in  this,  lines 
are  drawn  to  represent  the  manner,  in  which  we  may  imagine 
the  sound  to  be  reflected.  There  is  a  point  in  front  of  the  trum- 
pet, F,  which  is  denominated  its  focus,  because  the  sound  is 
there  more  intense,  than  at  any  other  spot.  The  trumpet  used 
by  deaf  persons,  acts  on  the  same  principle;  although  it  does 
not  equally  increase  the  sound. 

Emily.  Are  the  trumpets  used  as  musical  instruments,  also 
constructed  on  this  principle? 

Mrs.  B.  So  far  as  their  form  tends  to  increase  the  sound, 
they  are;  but,  as  a  musical  instrument,  the  trumpet  becomes  it- 
self the  sonorous  body,  which  is  made  to  vibrate  by  blowing  into 
it,  and  communicates  its  vibrations  to  the  air. 

I  will  attempt  to  give  you,  in  a  few  words,  some  notion  of  the 
nature  of  musical  sounds,  which,  as  you  are  fond  of  music,  must 
be  interesting  to  you. 

32.  How  are  echoes  produced  ?  33.  What  is  the  operation  and  effect  of 
the  speaking-trumpet  (fig.  7,  plate  14)  J' 


ON    WIND    AND    SOUND.  155 

If  a  sonorous  body  be  struck  in  such  a  manner,  that  its  vibra- 
tions, are  all  performed  in  regular  times,  the  vibrations  of 
the  air,  will  correspond  with  them;  and  striking  in  the  same 
regular  manner  on"  the  drum  of  the  ear,  will  produce  the 
same  uniform  sensation,  on  the  auditory  nerve,  and  excite  the 
same  uniform  idea,  in  the  mindj  or,  in  other  words,  we  shall 
hear  one  musical  tone. 

But  if  the  vibrations  of  the  sonorous  ho({y,  are  irregular,  there 
will  necessarily  follow  a  confusion  of  aerial  vibrations;  for  a 
second  vibration  may  commence,  before  the  first  is  finished,  meet 
it  half  way  on  its  return,  interrupt  it  in  its  course,  and  produce 
harsh  jarring  sounds,  which  are  called  discords, 

Emily.  But  each  set  of  these  irregular  vibrations,  if  repeated 
alone,  and  at  equal  intervals,  would,  I  suppose,  produce  a  musi- 
cal tone?  It  is  only  their  irregular  interference,  which  occasions 
discord. 

Mrs.  B.  Certainly.  The  quicker  a  sonwous  body  vibrates, 
the  more  acute,  or  sharp,  is  the  sound  produced;  and  the  slower 
the  vibrations,  the  more  grave  will  be  the  note. 

Caroline.  But  if  I  strike  any  one  note  of  the  piano-forte, 
repeatedly,  whether  quickly  or  slowly,  it  always  gives  the  same 
tone. 

Mrs.  B.  Because  the  vibrations  of  the  same  string,  at  the 
same  degree  of  tension,  are  always  of  a  similar  duration.  The 
quickness,  or  slowness  of  the  vibrations,  relate  to  the  single  tones, 
not  to  the  various  sounds  which  they  may  compose,  by  succeed- 
ing each  other.  Striking  the  note  in  quick  succession,  produces 
a  more  frequent  repetition  of  the  tone,  but  does  not  increase  the 
velocity  of  the  vibrations  of  the  string. 

The  duration  of  the  vibrations  of  strings,  or  wires,  depends 
upon  their  length,  their  thickness,  or  weight,  and  their  degree 
ot  tension:  thus,  you  find,  the  low  bass  notes  are  produced  by 
long,  thick,  loose  strings;  and  the  high  treble  notes  by  short, 
small,  and  tight  strings. 

Caroline.  Then,  the  different  length,  and  size,  of  the  strings 
of  musical  instruments,  serve  to  vary  the  duration  of  the  vibra- 
tions, and  consequently,  the  acuteness  or  gravity  of  the  notes? 

Mrs.  B.  Yes.  Among  the  variety  of  tones,  there  are  some 
which,  sounded  together,  please  the  ear,  producing  what  we  call 
harmony,  or  concord.  This  arises  from  the  agreement  of  the 
vibrations  of  the  two  sonorous  bodies;  so  that  some  of  the  vibra- 
tions of  each,  stilke  upon  the  ear  at  the  same  time.     Thus,  if  the 

34.  How  is  a  musical  tone  produced  ?  35.  What  occasions  discords  ?  36. 
Upon  what  does  the  acuteness  or  gravity  of  a  sound  depend?  37.  Does  the 
force,  with  which  a  string  is  struck,  affect  the  rapidity  of  its  vibrations  ?  38. 
How  are  the  strings  made  to  produce  the  high  and  low  notes  ?  39.  What  is 
meant  by  harmony,  or  concord,  and  how  is  it  produced.^ 


156  Oyf   WIND    AND    SOUN». 

vibrations  of  two  strings  are  performed  in  equal  times,  the  same 
tone  is  produced  by  both,  and  thej  are  said  to  be  in  unison. 

Emily,  Now,  tlien,  I  understand  why,  when  I  tune  my  harp, 
in  unison  with  the  piano-forte,  I  draw  the  strings  tighter,  if  it  is 
too  low,  or  loosen  them,  if  it  is  too  high  a  pitch:  it  is  in  order  to 
bring  them  to  vibrate,  in  equal  times,  with  the  strings  of  the 
piano-forte. 

Mrs,  B,  But  concord,  you  know,  is  not  confined  to  unison; 
for  two  different  tones,  harmonize  in  a  variety  of  cases.  When 
the  vibrations  of  one  string  (or  other  sonorous  body)  vibrate  in 
double  the  time  of  another,  the  second  vibration  of  the  latter,  will 
strike  upon  tlie  ear,  at  tlie  same  instant,  as  the  first  vibration  of 
the  former;  and  this  is  the  concord  of  an  octave. 

If  the  vibrations  of  two  strings  are  as  two  to  three,  the  second 
vibration  of  the  first,  corresponds  with  the  third  vibration  of  the 
latter,  producing  the  harmony  called,  a  fifth. 

Caroline.  So,  then,  when  I  strike  the  key-note  with  its  fifui, 
I  hear  every  second  vibration  of  one,  and  every  third  of  the 
other,  at  the  same  time? 

Mrs.  B.  Yes;  and  the  key-note,  struck  with  the  fourth,  is 
likewise  a  concord,  because  the  vibrations,  are  as  three  to  four. 
The  vibrations  of  a  major  third,  with  the  key-note,  are  as  four  to 
five;  and  those  of  a  minor  third,  as  five  to  six. 

There  are  other  tones,  which,  though  they  cannot  be  struck 
together  without  producing  discord,  if  struck  successively,  give 
us  that  succession  of  pleasing  sounds,  which  is  called  melody. 
Harmony,  you  perceive,  arises  from  the  combined  effect  of  two, 
or  more  concordant  sounds,  while  melody,  is  the  result  of  certain 
simple  sounds,  which  succeed  each  other.  Upon  these  general 
principles,  the  science  of  music  is  founded;  but,  I  am  not  suflB- 
ciently  acquainted  with  it,  to  enter  into  it  any  further. 

We  shall  now,  therefore,  take  leave  of  the  subject  of  sound; 
And,  at  our  next  interview,  enter  upon  that  of  optics,  in  which 
we  shall  consider  the  nature  of  light,  vision,  and  colours. 

40.  When  are  string;s  said  to  be  in  unison?  41.  How  are  octaTes  pro- 
duced? 42.  How  are  fifths  produced?  43.  How  major  and  minor  thirds  ? 
44.  What  is  meant  by  melodyj  and  in  what  particular  does  it  diffex  from  har- 
mony? 


^t^- 


Pl^teotv. 


CONVERSATION  XIV. 


ON  OPTICS. 

OF  LTTMUrorS,  TRANSPARENT,  AJfD    OPAaUE  BODIES. — OF  THE  RADIATION 
OF  LIGHT. — OF  SHADOWS. — OF  THE    REFLECTION  OF   LIGHT. — OPAaUE 

BODIES    SEEN    ONLY    BY     REFLECTED    LIGHT. — VISION    EXPLAINED. 

CAMERA  OBSCURA. — IMAGE  OF  OBJECTS  ON  THE  RETINA. 

CAROLINE. 

I  LONG  to  begin  our  lessoii  to  day,  Mrs.  B.,  for  I  expect  tbat 
it  will  be*  very  entertaining. 

•  Mrs.  B.  Optics  is  that  branch  of  philosophy,  which  treats  of 
the  nature  and  properties  of  light.  It  is  certainly  one  of  the 
most  interesting  branches  of  Natural  Pliilosophy,  but  not  one 
of  the  easiest  to  understand;  I  must,  therefore,  beg  that  you  will 
give  me  your  undivided  attention. 

I  shall  first  inquire,  whether  you  comprehend  tlie  meaning  o^ 
a  luminous  body,  an  opaque  body,  and  a  transparent  body. 

Caroline.     A  luminous  body  is  one  that  shines;  an  opaque.... 

Mrs.  B.  Do  not  proceed  to  the  second,  until  we  have  agreed 
upon  the  definition  of  the  first.  All  bodies  that  shine,  are  not 
luminous;  for  a  luminous  body  is  one  that  shines  by  its  own 
light;  as  the  sun,  the  fire,  a  candle,  &c. 

Emily.  Polished  metal  then,  when  it  shines  with  so  much 
brilliancy,  is  not  a  luminous  body? 

Mrs.  B.  No,  for  it  would  be  dark,  if  it  did  not  receive  light 
from  a  luminous  body;  it  belongs,  therefore,  to  the  class  of 
dark,  as  well  as  of  opaque  bodies,  which  comprehends  all  such  as 
are  neither  luminous,  nor  will  admit  the  light  to  pass  through  them. 

Emily.  And  transparent  bodies,  are  those  which  admit  the 
light  to  pass  through  them,  such  as  glass  and  water., 

Mrs.  B.  You  are  right.  Transparent,  or  pellucid  bodies, 
are  frequently  called  mediums,  because  they  allow  the  rays  of 
light  to  pass  through  them;  and  the  rays  which  pass  through, 
are  said  to  be  transmitted  by  them. 

Light,  when  emanated  from  the  sun,  or  any  other  luminous 

1.  What  is  optic3.?  2.  What  is  meant  by  a  luminous  body?  3.  What  is 
meant  by  a  dark  body,  and  what  by  an  opaque  body  ^  4.  What  are  transpa- 
rent bodies  ?    5.  What  is  a  medium  ? 

o 


ioB  ©N   Ol>Tl(iS. 

body,  is  projected  forward  in  strj^ight  lines,  in  every  possible 
direction^  so  that  the  luminous  body,  is  not  only  the  general 
centre, from  whence  all  the  rays  proceed;  but  every  point  of  it, 
may  be  considered  as  a  centre,  which  radiates  light  in  every  di- 
rection.  (Fig.  1,  plate  15.) 

Emily.  But  do  not  the  rays  which  are  projected  in  different 
directions,  and  cross  each  other,  interfere,  and  impede  each 
other's  course.^ 

Mrs,  B.  Not  at  all.  The  particles  of  light,  are  so  extreme- 
ly minute,  that  thev  are  never  known  to  interfere  with  each 
other.  A  ray  of  light,  is  a  single  line  of  light,  projected  from  a 
luminous  body;  and  a  pencil  of  rays,  is  a  collection  of  rays,  pro- 
ceeding from  any  one  point  of  a  luminous  body,  as  fig.  2» 

Caroline.  Is  light  then  a  substance  composed  of  particles, 
like  other  bodies.^ 

Mrs.  B.  That  is  a  disputed  point,  upon  which  I  cannot  pre- 
tend to  decide.  In  some  respects,  light  is  obedient  to  the  laws 
which  govern  bodies;  in  others,  it  appears  to  be  independent  of 
them:  thus,  though  its  course  is  guided  by  the  laws  of  motion, 
it  does  not  seem  to  be  influenced  by  those  of  gravity.  It  has 
never  been  discovered  to  have  weight,  though  a  variety  of  inte- 
resting experiments  have  been  made  with  a  view  of  ascertaining 
that  point;  but  we  are  so  ignorant  of  the  intimate  nature  of  light, 
that  an  attempt  to  investigate  it,  would  lead  us  into  a  labyrinth 
of  perplexity,  if  not  of  error;  we  shall,  therefore,  confine  our  at- 
tention to  those  properties  of  light,  which  are  well  ascertained. 

Let  us  return  to  the  examination  of  the  effects  of  the  radia- 
tion of  light,  from  a  luminous  ])ody.  Since  the  rays  of  light  are 
projected  in  straight  lines,  when  they  meet  with  an  opaque  body 
through  which  they  are  unable  to  pass,  they  are  stopped  short  in 
their  course;  for  they  cannot  move  in  a  curve  line  round  the  body. 

Caroline.  No,  certainly;  for  it  would  require  some  other 
force  besides  that  of  projection,  to  produce  motion  in  a  curve 
line. 

Mrs.  B.  The  interruption  of  the  rays  of  light,  by  the  opaque 
body,  produces,  therefore,  darkness  on  the  opposite  side  of  it; 
and  if^  this  darkness  fall  upon  a  wall,  a  sheet  of  paper,  or  any 
object  whatever,  it  forms  a  shadows 

Emily.  A  shadow,  then,  is  nothing  more  than  darkness  pro- 
duced by  the  intervention  of  an  opaque  body,  which  prevents 
the  rays  of  light  from  reaching  an  object  behind  it. 

6.  How  is  light  projected  from  lumiuous  bodies,  and  how,  from  every  point 
of  such  bodies,  (fi^.  1,  plate  15  ?)  7.  Why  do  not  the  rays  of  light  from  dif- 
ferent points,  stop  each  other's  progress  ?  8.  What  is  a  ray,  and  what  a  pen- 
cil of  rays?  fig.  2,  plate  15.  9.  Do  we  know  whether  light  is  a  substance, 
similar  to  bodies  in  general  ?  10.  When  a  ray  of  light  falls  upon  an  opaque 
body,  wUat  is  the  result .' 


ON    OPTICS. 


159 


Caroline.  Why  then  are  shadowg  of  different  degrees  of 
darkness^  for  I  should  have  supposed,  from  your  definition  of  a 
shadow,  that  it  would  have  been  perfectly  black? 

Mrs,  B.  It  frequently  happens  that  a  shadow  is  produced  by 
an  opaque  body,  interrupting  the  course  of  the  rays  from  ohe 
luminous  body,  while  light  from  another,  reaches  the  space  where 
the  shadow  is  formed;  in  which  case,  the  shadow  is  proportion- 
ally fainter.  This  happens  when  the  opaque  body  is  lighted  by 
two  candles:  if  you  extinguish  one  of  them,  the  shadow  will  be 
both  deeper,  and  more  distinct. 

Caroline,     But  yet  it  will  not  be  perfectly  dark. 

Mrs.  B.  Because  it  is  still  slightly  illuminated  by  light 
reflected  from  the  walls  of  the  room,  and  other  surroundmg 
objects.": 

You  must  observe,  also,  that  when  a  shadow  is  produced  by 
the  interruption  of  rays  from  a  single  luminous  body,  the  dark- 
ness is  proportioned  to  the  intensity  of  the  light. 

Emily.  I  should  have  supposed  the  contrary;  for  as  the 
light  reflected  from  surrounding  objects  on  the  shadow,  must  be 
in  proportion  to  the  intensity  of  the  light,  the  stronger  the  light, 
the  more  the  shadow  will  be  illumined. 

Mrs.  B.  Your  remark  is  perfectly  just;  but  as  we' have  no 
means  of  estimating  the  degrees  of  light,  and  of  darkness,  but  by 
comparison,  the  strongest  light  will  appear  to  produce  the  deep- 
est shadow.  Hence  a  total  eclipse  of  the  sun,  occasions  a  more 
sensible  darkness  than  midnight,  as  it  is  immediately  contrast- 
ed with  the  strong  light  of  noonday. 

Caroline.  The  re-appearance  of  the  sun,  after  an  eclipse, 
must,  by  the  same  contrast,  appear  remarkably  brilliant. 

Mrs.  -  B.  Certainly.  There  are  several  things  to  be  observed, 
in  regard  to  the  form,  and  extent,  of  shadows.  If  the  luminous 
body  A  (fig.  3.)  is  larger  than  the  opaque  body  B,  the  shadow 
will  gradually  diminisli  in  size,  till  it  terminates  in  a  point. 

Caroline.  This  is  the  case  with  the  shadows  of  the  earth,  and 
the  moon;  as  the  sun,  which  illumines  them,  is  larger  than  either 
of  those  bodies.  And  why  is  it  not  the  case  with  the  shadows 
of  teriestrial  objects.^  Their  shadows,  far  from  diminishing,  are 
alwa^  larger  than  the  object,  and  increase  with  the  distance 
from  it. 

Mrs.  B.  In  estimating  the  effect  of  shadows,  we  must  con- 
sider the  dimensions  of  the  luminous  body;  when  the  luminous 
body  is  less,  than  tlie  opaque  body,  the  shadow  will  increase 

11.  In  what  does  shadow  consist?  12.  Why  are  they,  in  general,  but 
partially  dark?  13.  Upon  what  does  the  intensity  of  a  shadow  depend? 
14.  How  are  shadows  affected  by  the  size  of  thfe  luminous  body,  as  represent- 
ed in  plate  15,  fig.  3  ?  15.  When  is  the  shadow  larger  than  the  intercepting- 
body  ? 


"160  On  optics. 

with  the  distance.  This  will  be  best  exemplified,  by  observini^- 
the  shadow  of  an  object  lighted  by  a  candle. 

Emily.  I  have  often  noticed,  that  the  shadow  of  my  figure, 
against  the  wall,  grows  larger,  as  it  is  more  distant  from  me, 
which  is  owing,  no  doubt,  to  the  candle  that  shines  on  me,  be- 
ing much  smaller  than  myself. 

Mrs.  B,  Yes.  The  shadow  of  a  figure  as  A,  (fig.  4.)  varies 
in  size,  according  to  the  distance  of  the  several  surfaces  BCD 
K,  on  which  it  is  described) 

CaroHne.  I  have  observed,  that  two  candles,  produce  two 
ivhadows  from  the  same  object;  wiiilst  it  would  appear,  from 
what  you  said,  that  they  should  ratlier  produce  only  half  a  sha- 
llow, that  is  to  say,  a  very  faint  one. 

Mrs.  B.  The  nftniber  of  lights  (in  diflerent  directions)  while 
it  decreases  the  intensity  of  the  shadows,  increases  their  number, 
whicli  always  corresponds  with  that  of  the  lights;  for  each  lights 
makes  the  opaque  body  cast  a  different  shadow,  as  illustrated  by 
iio'.  5.  whicli  represents  a  ball' A,  lighted  by  three  candles,  B, 
O,  D;  and  you  ol)serve  the  light  B,  produces  the  shadow  b,  the 
''ii;ht  C,  the  shadow  c,  and  the  light  D,  the  shadow  dj,  but  nei- 
hor  of  these  shadows  will  be  very  dark,  because  the  light  of  one 
iiudle  only,  is  intercepted  by  the  ball;  and  the  spot  is  still  illu- 
ninated  by  the  other  two.,' 

Emily.  I  think  we  now  underetand  the  nature  of  shadowy 
very  well;  but  pray,  what  becomes  of  the  rays  of  light,  which 
opaque  bodies  arrest  in  tlieir  course,  and  tlie  interruption  of 
which,  is  the  occasion  of  sliadows? 

Mrs.  B.  Your  question  leads  to  a  very  important  propert}^ 
<»f  light.  Reflection.  When  rays  of  light  encounter  an  opaque 
body,;  tliey  cannot  pass  through  it,  and  part  of  them  are  absorbed 
by  it,  and  part  are  reflected,  and  rebound;  just  as  an  elastic  ball 
rebounds,  when  struck  against  a  wall. 

By  reflection,  we  mean  that  the  light  is  turned  back  again, 
rhrouiiii  the  same  medium  which  it  had  traversed  in  its  first 


course. 


Emily.  And  is  light,  in  its  reflection,  governed  by  the  same 
laws,  as  solid,  elastic  bodies?  « 

Mrs.  B.  Exactly.  If  a  ray  of  light  fall  perpendicularly  on 
an  opaque  body,  it  is  reflected  baok  in  the  same  line,  towards 
the  point  whence  it  proceeded.  If  it  fall  obliquely,  it  is  reflect- 
ed obliquely,  but  in  the  opposite  direction;  the  ray  which  falls 
u])on  the  reflecting  surface,  is  called  the  incident  ray,  and  that 
^vhich  leaves  it,  the  reflected  ray;  the  angle  of  incidence,  is  al- 

16.  What  is  explained  by  fig.  4,  plate  15  .'*  17.  What  will  be  the  effect  of 
:>everal  lights,  as  in  fig.  5,  plate  15  ?  18.  Why  will  nffither  of  these  shadows 
be  very  dark?  19.  What  becomes  of  the  Ijght  which  falls  upon  an  opaque 
body  ?     20.  What  is  meant  by  reflection  ? 


ON   OPTICS.  161 

^rays^qual  to  the  angie  of  reflection.     You  recollect  that  law 
in  mechanics? 

Emily.    Oh  yes,  perfectly.  , 

Mrs.  B.  If  you  will  shut  the  shutters,  we  will  admit  a  ray 
of  the  sun's  light,  through  a  very  small  aperture,  and  I  can  show 
you  how  it  is  reflected.  I  now  hold  this  mirror,  so  that  the  ray 
shall  fall  perpendicularly  upon  it. 

Caroline.  I  see  the  ray  which  falls  upon  the  mirror,  but  not 
that  which  is  reflected  hj  it. 

Mrs.  B.  Because  it  is  turned  directly  back  again;  and  the 
ray  of  incidence,  and  that  of  reflection,  are  confounded  together, 
both  being  in  the  same  line,  though  in  opposite  directions. 

Emily.  The  ray  then,  which  appears  to  us  single,  is  really 
double,  and  is  composed  of  the  incident  ray,  proceeding  to  the 
mirror,  and  of  the  reflected  ray,  returning  from  the  mirror. 

Mrs.  B.  Exactly  so.  We  will  now  separate  them,  by  hold- 
ing the  mirror  M,  (ng.  6,)  in  such  a  manner,  that  the  incident 
ray,  A  B,  shall  fall  obliquely  upon  it— you  see  the  reflected 
ray,  B  C,  is  marching  oft'  in  another  direction.  If  we  draw  a 
line  from  the  point  of  incidejice  B,  perpendicularly,  to  the  mir- 
ror, it  will  divide  the  angle  of  incidence,  from  the  angle  of  re- 
flection, and  you  will  see  that  they  are  equal. 

Emily.  Exactly;  and  now,  that  you  hold  the  mirror,  so  that 
the  ray  falls  more  obliquely  upon  it,  it  is  also  reflected  more 
obliquely,  preserving  the  equality  of  the  angles  of  incidence,  and 
of  reflection. 

Mrs.  B.     It  is  by  reflected  rays  only,  that  we  see  opaque  ob 
jects.     Luminous  bodies,  send  rays  of  light  immediately  to  our 
eyes,  but  the  rays  which  they  send  to  other  bodies,  are  invisible 
to  us,  and  are  seen,  only  when  they  are  reflected  by  those  bo- 
dies, to  our  eyes. 

Emily.  But  have  we  not  just  seen  the  rjiy  of  light,  in  its  pass- 
age  from  the  sun  to  the  mirror,  and  its  reflections.'^  yet,  in  nei- 
ther case,  were  those  rays  in  a  direction  to  enter  our  eyes. 

Mrs.  B.  What  you  saw,  was  the  light  reflected  to  youi 
eyes,  by  small  particles  of  dust  floating  in  the  air,  and  on  which 
tne  ray  shone,  m  its  passage  to,  and  from,  the  mirror. 

Caroline.  Yet  I  see  the  sun,  shining  on  that  house  yonder, 
as  clearly  as  possible. 

Mrs.  B,     Indeed  you  cannot  see  a  single  ray,  which  passes 

21.  What  is  meant  by  the  incident,  and  reflected  rays?  22.  What  is  the 
result,  when  the  incident  ray  falls  perpendicularly,  and  what,  when  it  falls 
obliquely  ?  23.  What  two  angles  are  always  equal  in  this  case  ?  24.  To 
what  law  in  mechanics,  is  this  analogous,  as  represented  in  %.  4,  plate  2  ? 
25.  What  is  represented  by  fig.  6,  plate  15  ?  26.  By  what  light  are  we  ena- 
bled to  see  opaque,  and  by  what,  luminous  bodies  ?  27.  What  enables  us  to 
see  a  ray  of  light  in  its  passage,  through  a  darljened  room  i* 


162 


ON    OPTICS. 


from  the  sun  to  the  house;  jou  see,  by  the  aid  of  those  rays, 
wliich  enter  your  eyes;  therefore, (it  is  the  rays  which  are  re- 
flected by  the  house,  to  you,  and  not  those  which  proceed  di- 
rectly from  the  sun,  to  the  house,  that  render  the  building  visi- 
ble to  you.  > 

Caroline.  Why  then  does  one  side  of  the  house  appear  to  be 
in  sunshine,  and  the  other  in  shade?  for,  if  I  cannot  see  the  sun 
shine  upon  it^,  the  whole  of  the  house  should  appear  in  the  shade. 

Mrs.  B.  That  side  of  the  house,  which  the  sun  shines  upon, 
receives, and  reflects  more  light, and  therefore,  appears  more  lumi- 
nous and  vivid,'  than  tlie  side  which  is  in  shadow;  for  the  latter 
is  illumined  only,  by  rays  reflected  upon  it  by  other  objects; 
these  rays  are,  therefore,  twice  reflected  before  they  reach  your^ 
sight;  and  as  light  is  more,  or  less,  absorbed  by  the  bodies  it 
strikes  upon,  every  time  a  ray  is  reflected,  its  intensity  is  dimin- 
ished. 

Caroline.  Still  I  cannot  reconcile  to  myself,  the  idea  that  we 
do  not  see  the  sun's  rays  shining  on  objects,  but  only  those  which 
such  objects  reflect  to  us. 

Mrs.  B.  I  do  not,  however,  despair  of  convincing  you  of  it. 
Look  at  that  large  sheet  of  water;  can  you  tell  why  the  sun  ap- 
pears to  shine  on  one  part  of  it  only.'^ 

Caroline.  No,  indeed;  for  the  whole  of  it  is  equally  exposed 
to  the  sun.  This  partial  brilliancy  of  water,  has  often  excited 
my  wonder;  but  it  has  struck  me  more  particularly  by  moon- 
light. I  have  frequently  observed  a  vivia  streak  of  moonshine 
oil  the  se^,  while  the  rest  of  the  water  remained  in  deep  obscu- 
rity, and  yet  there  was  no  apparent  obstacle  to  prevent  the  moon 
from  shining  equally  on  every  part  of  the  water. 

Mrs.  B.  By  moonlight  the  effect  is  more  remarkable,  on 
account  of  the  deep  obscurity  of  the  other  parts  of  the  water; 
while  by  the  sun's  light,  the  effect  is  too  strong  for  the  eye  to  be 
able  to  observ^e  it  so  distinctly. 

Caroline.  But,  if  the  sun  really  shines  on  every  part  of  that 
sheet  of  water,  why  does  not  every  part  of  it,  reflect  rays  to  my 
eyes?    * 

Mrs.  B.  The  reflected  rays,  ai-e  not  attracted  out  of  their 
natural  course,  by  your  eyes.  The  direction  of  a  reflected  ray, 
you  know,  depends  on  that  of  the  incident  ray;  the  sun's  ray's, 
therefore,  which  fall  with  various  degrees  of  obliquity  upon  the 
water,  are  reflected  in  directions  equally  various;  some  of  these 

28.  By  what  reasoning  would  you  prove  that  an  object,  such,  for  exaniple, 
as  a  house,  is  seen  by  reflected  light  ?  29.  Why  may  one  side  of  such  object 
appear  more  bright  than  another  side?  30.  How  is  the  fact  exemplified  by 
the  sun,  or  moon,  shining  upon  water  f  31 .  Why  is  this  best  evinced  by  moon- 
lisht.? 


ON    OPTICS. 


will  meet  your  eyes,  and  you  will  see  tUem,  but  those  which  fall 
'''^«;"'^'^retlk  rs;a.hi.e,  then  wM 
upo^rU^Twate.,  is  composed  of  those  rays  which  by  then-  reflec- 
tion, happen  to  fall  upon  my  eyes? 

tot^u  ;hadow,  really  illuminated  by  the  sun,  and  its  rays  re- 

""'X  T^N^Sat  is  a  different  case,  from  the  sheet  of  wa- 

ter      Tim    side  of  the  house,is  really  in  shadow;  it  is  the  west 

si,  wMch  the  sun  cannot  shine  upon,  tdllhe  at™-„^^^^^ 

£„%.     Those  objects,  tl.en,  w  neh  are  '"""""^^  t-J  '^^^^^^^^ 

x/htrr^Shri^s^i^wU:^^^^^^^^ 
3^..^^r'^Lf^r:he^-^|r:ti^- 

trees  cast  a  shadow,  by  what  light  do  you  see  itr 

Emilv      Since  it  is  not  by  the  sun's  direct  rays,  it  must  De  oy 
thofe  reflected  on  it  from  other  objects,  and  which  it  again  re- 

^'fJoZ'  But  if  we  see  all  terrestrial  objects  by  reflected 
lio-ht  ?a  we  do  the  moon,)  why  do  they  appear  so  tnght  and 
Slious?  I  should  have''s«pposed  that  reflected  rays,  would 
have  been  dull  and  faint,  like  those  of  the  moon. 

Mn  B.  The  moon  reflects  the  sun's  ight,  with  as  much 
vividness  as  any  terrestrial  object  If  you  look  at  it  on  a  clear 
nlht  It  will  appear  as  bright  as  a  sheet  of  water,  the  walls  of  a 
house,  or  any  object  seen  by  daylight,  and  on  whicji  tlie  sun 
sZes.  The  rays  of  tlie  moon  are  Soubtless  feeble,  when  com- 
pared with  those  of  the  sun;  but  that  would  not  be  a  fair  com- 
parfsonTfor  the  former  are  incident,  the  latter,  reflected  rays 
^  Mine  True;  and  when  we  see  terrestiial  objects  by 
mLu?5!theu|ht  has  been  twice  reflected,  and  is  consequent- 

^^'ifi-rs.'^iS^'Tn^ravtrsing  thl  a.ft.r ^osphere,  the  rays,  both  of  the 
sun,  and  moon,  lose  some  of  their  light.  ^  For  though  the  pure 
air,  is  a  transparent  medium,  which  transmits  flie  .  -^ys  oi  iignt 
freelv,  we  have  observed,  that  near  the  surface  of  the  ea. ..  "  n,  it  i» 
loaded  with  vapours  and  exhalations,  by  which  some  portion  t  ' 
them 'are  absorbed.  ,  .    ,       x. 

Caroline.     I  have  often  noticed,  that  an  object  on  the  summit 

32.  By  what  light  do  we  see  the  moon,  and  why  is  it  comparatively  fee-  ^ 
ble  ?    33.  What  eiroumstance,  renders  objects  seen  by  moonlight,  still  less 
vivid  ? 


164 

ON    OPTICS. 


Mrs.  B.     That  may  have  some  sensible  effprf.  hn+       u 

Mrs.  B     I  shall  hereafter  describe  thi  strucW  „f  tl.« 
very  particularly,  but  will  „„w  observrihat  the  smll    rn?^*l 
spot,  which  IS  generally  called  the  sio-ht  of  X  „f     ■  "',"' 

.lenominated  th'e  ;,,,;.-/and  thtt  the  X^  t  ex^L"  „Wj 
optic  nerve  on  the  back  part  of  tlie  ball  of  thepl?  l\^ 

ra\rf".Hr7  ^  IP  ^^'!'  "h-t'Lte*v^:';q/''^h^; 

will  be  rendered  very  distinct  j?  i"  ^  locus,  it 

about  by  the  wind.     The  landscape,  would  be  perfectTit  weTe 
i^?^.  i/.     It  IS  not  enough  to  admire,  you  must  understand 

eSil  Thf'  "'''^"',^  of  darkening  the  room/fn  order  to 
exhibit  It.  The  camera  obscura,  sometimes  consists  of  a  sm.ll 
box,  properly  fitted  up,  to  represent  external  objects! 

refl^c;:crtor^^^^^^  ^^  ti.  ....^.^^^^^^ 

admitted  t^^^^^^^^^^^  and  which  are 

alcove  A'^;./''?'^J>^^  ft«  glittering  weathercock,  at  the  top  of  the 

cock   >•    '''  ^^l^P^ate  16.)  represent  it  in  this  spot,  a;  for  the  weather- 

.-.  tlore,  \ieing  much  higher  than  the  aperture  in  the  shutter,  only 

water  tew  of  the  rays,  which  are  reflected  by  it,  in  an  obliquely  de- 

cending  direction,  can  find  entrance  there.     The  rays  of  light, 

^^  ^  o.>^tv^^^*  "  *"®^*  ^y  *^®  P"P^^  °^  ^^^  ®y®-^  35.  What  by  the  retina  r 
app'  36.  How  do  the  rays  of  light  operate  on  the  eye  in  producing  vision?  37. 
the  How  may  this  be  exemplified,  in  a  darkened  room  ?  38.  What  is  meant  by 
iigr    a  camera  obscura?    39.  How  is  it  explained  in  plate  IG  ' 


ON    OPTICS*  165 

you  know,  always  move  in  straight  lines;  those,  therefore,  which 
enter  the  room,  in  a  descending  direction,  will  continue  their 
course  in  the  same  direction,  and  will  consequently  fall  upon 
the  lower  part  of  the  wall  opposite  the  aperture,  and  represent 
the  weathercock,  reversed  in  that  spot,  instead  of  erect,  in  the 
uppermost  part  of  the  landscape. 

t^mily.     And  the  rays  of  light,  from  the  steps,  (B)  r^'  ^ne. 
alcove,  in  entering  the  aperture,  ascend,  and  will  descm6e  those 
steps  in  the  highest,  instead  of  the  lowest,  part  of  th«  landscape. 

Mrs.  B,  Observe,  too,  that  the  rays  comi?A-  iiom  the  alcove, 
which  is  to  our  left-,  d-t^crfbe  it  on  the  wall,  to  the  right;  while 
those,  whi^-t-  are  reflected  by  the  walnut  tree,  C  D,  to  our  right, 
t'l'Jiineate  its  figure  in  the  picture,  to  the  left,  c  d.  Thus  the 
rays,  coming  in  different  directions,  and  proceeding  always  in 
right  lines,  cross  each  other  at  their  entrance  through  the  aper- 
ture; those  which  come  from  above,  proceed  below,  those  from  the 
right,  go  to  the  left,  those  from  the  left,  towards  the  right;  thus 
every  object  is  represented  in  the  picture,  as  occupying  a  situa- 
tion, the  very  reverse  of  that  which  it  does  in  nature. 

Caroline.  Excepting  the  flower-pot,  E  F,  which,  though  its 
position  is  reversed,  has  not  changed  its  situation  in  the  land- 
scape. 

Mrs.  B.  The  flower-pot,  is  directly  in  front  of  the  aperture; 
so  that  its  rays,  fall  perpendicularly  upon  it,  and  consequently 
proceed  perpendicularly  to  the  wall,  where  they  delineate  the 
object,  directly  behind  the  aperture. 

Emily.  And  is  it  thus,  that  the  picture  of  objects,  is  painted 
on  the  retina  of  the  eye? 

Mrs.  B.  Precisely.  The  pupil  of  the  eye,  through  which 
the  rays  of  light  enter,  represents  the  aperture  in  the  window- 
shutter;  and  the  image,  delineated  on  the  retina,  is  exactly 
similar  to  the  picture  on  the  wall. 

Caroline.  You  do  not  mean  to  say,  that  we  see  only  the  re- 
presentation of  the  object,  which  is  painted  on  the  retina,  and 
not  the  object  itself? 

Mrs.  B.  If,  by  sight,  you  understand  that  sense,  by  which 
tJie  presence  of  objects  is  perceived  by  the  mind,  through  the 
means  of  the  eyes,  we  certainly  see  only  the  image  of  those  ob- 
jects, painted  on  the  retina. 

Caroline.     This  appears  to  me  quite  incredible. 

Mrs.  B.  ^he  nerves,  are  the  only  part  of  our  frame,  capable 
of  sensation:  they  appear,  therefore,  to  be  the  instrumeMts, 
which  the  mind  employs  in  its  perceptions;  for  a  sensation,  al- 

40.  Why  are  the  objects  inverted  and  reversed?  41.  What  analogy  is 
there  between  the  camera  ob§cura,  and  the  eye  ?  42.  Is  it  the  object,  or  its 
pictiu-e  on  tlie  retina,  which  presents  to  tlie  mind  an  idea  of  the  object  seen  i* 


166 


ON    OPTICS. 


ways  conveys  an  idea,  to  the  mind.  Now  it  is  known,  that  our 
nerves  can  be  affected  only  by  contact,*  and  for  this  reason,  the 
organs  of  sense,  cannot  act  at  a  distance:  for  instance,  we  are 
capable  of  smelling  only  particles  which  are  actually  in  contact 
with  the  nerves  ot  the  nose.  We  have  already  observed,  that 
the  odour  of  a  flower  consists  in  effluvia,  composed  of  very  mi- 
nute particles,  which  penetrate  the  nostrils,  and  strike  upon  the 
olfactory  nerves,  which  instantly  convey  the  idea  of  odour  to  the 
mind. 

Emily.  And  sound,  though  it  is  said  to  be  heard  at  a  dis- 
tance, is,  in  fact,  heard  only  when  the  vibrations  of  the  air, 
which  convey  it  to  our  ears,  strike  upon  the  auditory  norve. 

Caroline,  There  is  no  explanation  required,  to  prove  that  the 
senses  of  feeling  and  of  tasting,  are  excited  only  by  contact. 

Mrs,  B.  And  I  hope  to  convince  you,  that  the  sense  of  sight, 
is  so  likewise.  The  nerves,  which  constitute  the  sense  of  sight, 
are  not  different  in  their  nature  from  those  of  the  other  organs,* 
they  are  merely  instruments  which  convey  ideas  to  the  mind, 
and  can  be  affected  only  on  contact.  Now,  since  real  objects 
cannot  be  brought  to  touch  the  optic  nerve,  the  image  of  them  is 
conveyed  thither  by  the  rays  of  light,  proceeding  from  real  ob- 
jects, which  actually  strike  upon  the  optic  nerve,  and  form  that 
image  which  the  mind  perceives. 

Caroline.  While  I  listen  to  your  reasoning,  I  feel  convinced; 
but  when  I  look  upon  the  objects  around,  and  think  that  I  do 
not  see  them,  but  merely  their  image  painted  in  my  eyes,  my 
belief  is  again  staggered.  I  cannot  reconcile  to  myself,  the  idea, 
that  I  do  not  really  see  this  book  which  I  hold  in  my  hand,  nor 
the  words  which  I  read  in  it. 

Mrs.  B,  Did  it  ever  occur  to  you  as  extraordinary,  that  you 
never  beheld  your  own  face.^ 

Caroline.  No;  because  I  so  frequently  see  an  exact  repre- 
sentation of  it  in  the  looking-glass. 

Mrs.  B.  You  see  a  far  more  exact  representation  of  objects 
on  the  retina  of  your  eye:  it  is  a  much  more  perfect  mirror,  than 
any  made  by  art. 

Emily.  But  is  it  possible,  that  the  extensive  landscape,  which 
I  now  behold  from  the  window,  should  be  represented  on  so 
small  a  space,  as  the  retina  of  the  eye? 

Mrs.  B.  It  would  be  impossible  for  art  to  paint  so  small 
and  distinct  a  miniature;  but  nature  works  with  a  surer  hand, 
and  a  more  delicate  pencil.  That  power  alone,  which  forms  the 
feathers  of  the  butterfly,  and  the  organs  of  the  minutest  insect,  can 

43.  By  what  organs  is  sensation  produced,  and  how  must  these  organs 
be  affected?  44.  How  will  the  idea  of  contact,  apply  to  objects  not  torching 
the  eye  ? 


ON   OPTICS.  167 

pourtray  so  admirable  and  perfect  a  miniature,  as  that  which  is 
it  presented  on  the  retina  of  the  eye. 

Caroline.  But,  Mrs.  B.,  if  we  see  only  the  image  of  objects, 
why  do  we  not  see  them  reversed,  as  you  showed  us  they  were, 
in  the  camera  obscura.^  Is  not  that  a  strong  argument  against 
your  theory? 

Mrs.  B.  Not  an  unanswerable  one,  I  hope.  The  image 
on  the  retina,  it  is  true,  is  reversed,  like  that  in  the  camera  ob- 
scura;  as  the  rays,  from  the  difterent  parts  of  the  landscape,  in- 
tersect each  other  on  entering  the  pupil,  in  the  same  manner  as 
they  do,  on  entering  the  camera  obscura.  The  scene,  however, 
does  not  excite  the  idea  of  being  inverted,  because  we  always  see 
an  object  in  the  direction  of  the  rays  which  it  sends  to  us. 

Emily.     I  confess  I  do  not  understand  that. 

Mrs.  B.  It  is,  I  think,  a  difficult  point  to  explain  clearly. 
A  ray  which  comes  from  the  upper  part  of  an  object,  describes 
the  image  on  the  lower  part  of  the  retina;  but,  experience  having 
taught  us,  that  the  direction  of  that  ray  is  from  above,  we  con- 
sider that  part  of  the  object  it  represents  as  uppermost.  The 
rays  proceeding  from  the  lower  part  of  an  object,  fall  upon  the 
upper  part  of  tiie  retina;  but  as  we  know  their  direction  to  be 
from  below,  we  see  that  part  of  the  object  they  describe  as  the 
lowest. 

Caroline.  "When  I  want  to  see  an  object  above  me,  I  look 
up;  when  an  object  below  me,  I  look  down.  Does  not  this 
prove  that  I  see  the  objects  themselves?  for  if  I  beheld  only  the 
image,  there  would  be  no  necessity  for  looking  up  or  down,  ac- 
cording as  the  object  was  higher  or  lower,  than  myself. 

Mrs.  B.  1  be^  your  pardon.  When  you  look  up,  to  an  ele- 
vated object,  it  is  in  order  that  the  rays  reflected  from  it,  should 
fall  upon  the  retina  of  your  eyes;  but  the  very  circumstance  of 
directing  your  eyes  upwards,  convinces  you  that  the  object  is 
elevated,  and  teaches  you  to  consider  as  uppermost,  the  image  it 
forms  on  the  retina,  though  it  is,  in  fact,  represented  in  the 
lowest  part  of  it.  When  you  look  down  upon  an  object,  you 
draw  your  conclusion  from  a  similar  reasoning;  it  is  thus  that 
we  see  all  objects  in  the  direction  of  the  rays  which  reach  our 
eyes. 

But  I  have  a  further  proof  in  favour  of  what  I  have  advanced, 
which,  I  hope,  will  remove  your  remaining  doubts:  I  shall,  how- 
ever, defer  it  till  our  next  meeting,  as  the  lesson  has  been  suffi- 
ciently long  to-day. 

45.  Why  do  not  objects  appear  reversed  to  the  eye,  at  in  the  camera  ^ 
scura  ? 


CONVERSATION  XV. 


OYTlCS—eontinued, 

ON  THK  ANGLE  OF  VISION,  AND  THE  REFLECTION  OF 
MIRRORS. 

-INGLE   OF   VISION. — REFLECTION  OF  PLAIN   MIRRORS. — REFLECTION   OF 
CONVEX   MIRRORS. — REFLECTION    OF   CONCAVE   MIRRORS. 

CAROLINE. 

Well,  Mrs.  B.,  I  am  very  impatient  to  hear  what  further 
proofs  yoii  have  to  oifer,  in  support  of  your  theory.  You  must 
allow,  that  it  was  ratlier  provoking  to  dismiss  us  as  you  did  at 
our  last  meeting. 

3Irs.  B.  You  press  so  hard  upon  me  with  your  objections, 
that  you  must  give  me  time  to  recruit  my  forces. 

Can  you  tell  me,  Caroline,  why  objects  at  a  distance,  appear 
smaller  than  they  really  are? 

Caroline,     I  know  no  otlier  reason  than  their  distance. 

Mrs.  B,  It  is  a  fact,  that  distance  causes  objects  to  appear 
smaller,  but  to  state  the  fact,  is  not  to  give  the  reason.  We  must 
refer  again  to  the  camera  obscura,  to  account  for  this  circum- 
stance; and  you  will  find,  that  the  difterent  apparent  dimen- 
sions of  objects  at  difterent  distances,  jTToceed  from  our  seeing, 
not  the  objects  themselves,  but  merely  their  image  on  the  re- 
tina. Fig.  1,  plate  17,  represents  a  row  of  trees,  as  viewed  in 
the  camera  obscura.  I  have  expressed  the  direction  of  the  rays, 
from  the  objects  to  the  image,  by  lines.  Now,  observe,  the  ray 
which  comes  from'  the  top  of  the  nearest  tree,  and  that  which 
comes  from  the  foot  of  the  same  tree,  meet  at  the  aperture,  form- 
ing an  angle  of  about  twenty-five  degrees;  the  angle  under 
which  we  see  any  object,  is  called,  the  visual  angle,  or,  angle  of 
vision.  These  rays  cross  each  other  at  the  aperture,  forming' 
equal  angles  on  each  side  of  it,  and  represent  the  tree  inverted 
in  the  camera  obscura.  The  degrees  of  the  image,  are  consider- 
ably smaller  than  those  of  the  object,  but  the  proportions  are 
perfectly  preserved. 

1.  What  is  meant  by  the  angle  of  vision,  or  the  risujil  angle  f 


Plate  3SVM. 


^^^ 


^    \ 


%k^"^i^sr^v~-i 


ON    THK    ANGLE    OF    VISION.  16l> 

Now,  let  US  notice  the  upper  and  lower  ray,  from  the  most 
distant  tree;  they  form  an  angle  of  not  more  tiian  twelve  or  fif- 
teen degrees,  and  an  image  of  proportional  dimensions.  Thus, 
two  objects  of  the  same  size,  as  the  two  trees  of  the  avenue, 
form  figures  of  diff'erent  sizes  in  the  camera  obscura,  according 
to  their  distance;  or,  in  other  words,  according  to  tlie  angle  of 
vision  under  which  they  are  seen.     Do  you  understand  this? 

Caroline.     Perfectly. 

Mrs.  Ri  Then  you  have  only  to  suppose,  that  the  represen- 
tation in  the  camera  obscura,  is  similar  to  that  on  the  retina. 

Now,  since  objects  of  the  same  magnitudes,  appear  to  be  of 
ditterent  dimensions,  when  at  different  distances  from  us,  let 
me  ask  you  which  it  is,  that  you  see;  the  real  objects,  which,  we 
know,  do  not  vary  in  size,  or  the  images,  which,  we  know,  do 
vary,  according  to  the  angle  of  vision  under  which  we  see  them? 

Caroline.  1  must  confess,  that  reason  is  in  favour  of  the  lat- 
ter. But  does  that  chair,  at  the  further  end  of  the  room,  form  an 
image  on  my  retina,  much  smaller  than  this  which  is  close  to 
me?  they  appear  exactly  of  the  same  size. 

Mrs.  B.  Our  senses  are  imperfect,  but  the  experience  we 
acquire  by  the  sense  of  touch,  corrects  the  illusions  of  our  siglit, 
with  regard  to  objects  within  our  reach.  You  are  so  perfectly 
convinced,  of  the  real  size  of  objects,  which  you  can  handle, 
that  you  do  not  attend  to  the  apparent  difference. 

Does  that  house  appear  to  you  much  smaller,  than  when  you 
are  close  to  it? 

Caroline.     No,  because  it  is  very  near  us. 

-Mrs.  B.  And  yet  you  can  see  the  whole  of  it,  through  one 
of  the  windows  of  this  room.  The  image  of  the  house  on  your 
retina  must,  therefore,  be  smaller  than  that  of  the  window 
through  which  you  see  it.  It  is  your  knowledge  of  the  real  mag- 
nitude of  the  house  which  prevents  your  attending  to  its  appa- 
rent size.  If  you  were  accustomed  to  draw  from  nature,  you 
would  be  fully  aware  of  this  difference. 

Emily.  And  pray,  what  is  the  reason  that,  when  we  look  up 
an  avenue,  the  trees  not  only  appear  smaller  as  they  are  more 
distant,  but  seem  gradually  to  approach  each  other,  till  they 
meet  in  a  point? 

Mrs.  B.  Not  only  the  trees,  but  the  road  which  separates 
the  two  rows,  forms  a  smaller  visual  an^le,  in  proportion  as  it 
is  more  distant  from  us;  therefore,  the  width  of  the  road  gradu- 
ally diminishes,  as  well  as  the  size  of  the  trees,  till  at  length  the 

2.  Why  do  objects  of  the  same  size  appear  smaller  when  distant,  than  when 
near?  3.  Why  do  not  two  object?,  known  to  be  equal  in  size,  appear  to 
differ,  when  at  different  distances  f'rou^  tU  eye  ?  4.  How  is  this  exemplified, 
by  a  house  seen  thiough  a  window: 

P 


170  i)N    THE    ANGLE    OF    VISION. 

road  apparently  terminates  in  a  point,  at  which  the  trees  seem 
to  meet. 

Emily.  I  am  very  glad  to  understand  this,  for  I  have  lately 
begun  to  learn  perspective,  which  appeared  to  me  a  very  dry 
study;  but  now  tnat  I  am  acquainted  with  some  of  the  principles 
on  which  it  is  founded,  I  shall  find  it  much  more  interesting. 

Caroline.  In  drawing  a  view  from  nature,  it  seems  that  we 
do  not  copy  the  real  objects,  but  the  image  they  form  on  the 
retina  of  our  eyes? 

Mrs.  B,  Certainly.  In  sculpture,  we  copy  nature  as  she 
really  exists;  in  paintmg,  we  represent  her,  as  she  appears  to  us. 

We  must  now  conclude  the  observations  that  remain  to  be 
made,  on  the  angle  of  vision. 

If  the  rays,  proceeding  from  the  exti-emities  of  an  object,  with 
an  ordinary  degree  of  illumination,  do  not  enter  the  eye  under 
an  angle  of  more  than  two  seconds,  which  is  the  1- 1800th  part 
of  a  degree,  it  is  invisible.  There  are,  consequently,  two  cases 
in  which  objects  may  be  invisible;  if  they  are  either  so  small, 
or  so  distant,  as  to  form  an  angle  of  less  than  two  seconds  of  a 
ilegree. 

In  like  manner,  if  the  velocity  of  a  body  does  not  exceed  20 
degrees  in  an  hour,  its  motion  is  imperceptible. 

Caroline.  A  very  rapid  motion  may  then  be  imperceptible, 
provided  the  distance  of  the  moving  body,  is  sufficiently  great. 

Mrs.  B.  Undoubtedly;  for  the  greater  it&  distance,  the 
smaller  will  be  the  an^le,  under  which  its  motion  will  appear 
to  the  eye.  It  is  for  this  reason,  that  the  motion  of  the  celestial 
bodies  is  invisible,  although  inconceivably  rapid. 

Emily.  I  am  surprised,  that  so  great  a  velocity  as  20  degrees 
an  hour,  should  be  invisible. 

Mrs.  B.  The  real  velocity  depends  upon  the  space  compre- 
hended in  each  degree,  and  upon  the  time,  in  which  the  moving 
body,  passes  over  that  space.  But  we  can  only  know  the  ex- 
tent of  this  space,  by  knowing  the  distance  of  the  moving  body, 
from  its  centre  of  motion;  for  supposing  two  men  to  set  off  at  the 
same  moment  from  A  and  B,  (fig.  2.^  to  walk  each  to  the  end 
of  their  respective  lines,  C  and  D;  if  they  perform  their  walk  in 

5.  Why  do  rows  of  trees,  forming  an  avenue,  appear  to  approach  as  they 
recede  from  the  eye,  until  they  eventually  seem  to  meet?  6.  In  drawing  a 
view  from  nature,  what  do  we  copy  ?  7.  What  is  the  diiference  in  sculpture, 
in  this  respect?  8.  Excepting  the  rays  from  an  object  enter  the  eye,  under 
a  certain  angle,  they  caimot  be  seen ;  what  must  this  angle  exceed  ?  9.  What 
two  circumstances  may  cause  the  angle  to  be  so  small,  as  not  to  produce  vi- 
sion? 10.  Motion  may  be  so  slow  as  to  become  imperceptible,  what  is  said 
on  this  point?  11.  Under  what  circumstances  may  a  body,  moving  with 
great  rapidity,  appear  to  be  at  rest?  12.  Upon  what  does  the  real  velocity 
of  a  body,  depend? 


ON    THE    ANGLE    OF    VISION.  171 

the  same  space  of  time,  they  must  have  proceeded  at  a  very  dif- 
ferent rate;  and  yet  to  an  eye  situated  at  E,  they  will  appear  to 
have  moved  witn  equal  velocity,  because  they  will  botli  have 
gone  through  an  equal  number  of  degrees,  though  over  a  very 
unequal  length  of  ground.  The  number  of  de^ees  over  whicli 
a  body  moves  in  a  given  time,  is  called  its  angular  velocity;  two 
bodies,  you  see,  may  have  the  same  angular,  or  apparent  velo- 
city, whilst  their  real  velocities  may  differ  almost  infinitely. 
Sight  is  an  extremely  useful  sense,  no  doubt,  but  it  cannot  al- 
ways be  relied  on,  it  deceives  us  both  in  regard  to  the  size  and 
the  distance  of  objects;  indeed,  our  senses  would  be  very  liable 
to  lead  us  into  error,  if  experience  did  not  set  us  right. 

Emily.  Between  the  two,  I  think  that  we  contrive  to  acquire 
a  tolerably  accurate  idea  of  objects. 

Mrs.  B.  At  least  sufficiently  so,  for  the  general  purposes  of 
life.  To  convince  you  how  requisite  experience  is,  to  correct 
the  errors  of  sight,  I  shall  relate  to  you,  the  case  of  a  young 
man,  who  was  blind  from  his  infancy,  and  who  recovered  his 
sight  at  the  age  of  fourteen,  by  the  operation  of  couching.  At 
first,  he  had  no  idea,  either  of  the  size,  or  distance  of  objects, 
but  imagined  that  every  thing  he  saw  touched  his  eyes;  and  it 
was  not,  till  after  having  repeatedly  felt  them,  and  walked 
from  one  object  to  another,  that  he  acquired  an  idea  of  their 
respective  dimensions,  their  relative  situations,  and  their  dis- 
tances. 

Caroline.  The  idea  that  objects  touched  his  eyes,  is,  how- 
ever, not  so  absurd,  as  it  at  first  appears;  for  if  we  consider  that 
we  see  only  the  image  of  objects,  this  image  actually  touches 
our  eyes. 

Mrs.  B.  That  is,  doubtless,  the  reason  of  the  opinion  he 
formed,  before  the  sense  of  touch  had  corrected  his  judgment. 

Caroline.  But  since  an  image  must  be  formed  on  the  retina 
of  each  of  our  eyes,  why  do  we  not  see  objects  double? 

Mrs.  B.  The  action  of  the  rays,  on  the  optic  nerve  of  each 
eye,  is  so  perfectly  similar,  that  they  produce  but  a  single  sen- 
sation; the  mind,  therefore,  receives  the  same  idea,  from  the 
retina  of  both  ej  es,  and  conceives  the  object  to  be  single. 

Caroline,  This  is  difficult  to  comprehend,  and  I  should  think, 
can  be  but  conjectural. 

Mrs.  B.  I  can  easily  convince  you,  that  you  have  a  distinct 
image  of  an  object  formed  on  the  retina  of  each  eye.     Look 

13.  What  must  be  known,  to  enable  us  to  ascertain  the  real  space  contain- 
ed in  a  degree?  14.  What  is  explained  by  %.  2,  plate  17?  15.  What  is 
said  respecting  the  evidence  afforded  by  our  senses,  and  how  do  we  correct  th€ 
errors  into  which  they  would  lead  us  ?  16.  An  image  of  a  visible  object  is 
formed  upon  the  retina  of  each  eye,  why,  therefore,  are  not  objects  seers 
double  ? 


irS  REFLECTION  OF  MIRRCRS. 

through  the  window,  with  botli  ejes  open,  at  some  object  exactly 
opposite  to  one  of  the  upright  bars  of  the  sash. 

Caroline.  I  now  see  a  tree,  the  body  of  which,  appears  to  be 
in  a  line  exactly  opposite  to  one  of  the  bars. 

Mrs.  B.  If  you  now  shut  your  right  eye,  and  look  with  the 
left,  it  will  appear  to  the  left  of  the  bar;  then  liy  closing  the  left 
eye,  and  looking  with  the  other,  it  will  appear  to  the  right  of 
tne  bar.' 

Caroline.     That  is  true,  indeed! 

Mrs.  B.  There  are,  evidently,  two  representations  of  the 
tree  in  different  situations,  which  must  be  owing  to  an  image  of 
^  it  being  formed  (m  each  eye;  if  the  action  of  the  rays,  there- 
Tore,  on  each  retina,  were  not  so  perfectly  similar  as  to  produce 
but  one  sensation,  we  should  see  double;  and  we  find  that  to  be 
the  case  with  some  persons,  who  are  afflicted  with  a  disease  in 
one  eye,  which  prevents  the  rays  of  light  from  affecting  it  in  the 
same  manner  as  the  other. 

Emily.  Pray,  Mrs.  B.,  when  we  see  the  image  of  an  object 
in  a  looking-glass,  why  is  it  not  inverted,  as  in  the  camera  ob- 
scura,  and  on  the  retina  of  the  eye.^ 

Mrs.  B.  Because  tlie  rays  do  not  enter  the  mirror  by  a 
small  aperture,  and  cross  each  other,  as  they  do  at  the  orifice 
of  a  camera  obscura,  or  the  pupil  of  the  eye. 

When  you  view  yourself  in  a  mirror,  the  rays  from  your  eyes 
fall  perpendicularly  upon  it,  and  are  reflected  in  the  same  line; 
the  image  is,  therefore,  described  behind  the  glass,  and  is  situ- 
ated in  tiie  same  manner  as  the  object  before  it. 

Emily.  Yes,  I  see  that  it  is;  but  the  looking-glass  is  not 
nearly  so  tall  as  I  am,  how  is  it,  therefore,  that  I  can  see  the 
A\'liole  of  my  figure  in  it.^ 

3Irs.  B.  It  is  not  necessary  that  the  mirror  should  be  more 
than  half  your  height,  in  order  that  you  may  see  the  whole  of 
your  person  in  it,  (tig.  3.)  The  ray  of  light  A  B,  from  your  eye, 
which  falls  perpendicularly  on  the  mirror  B  I),  will  be  reflected 
back,  in  the  same  line;  but  the  ray  from  your  feet,  will  fall  ob- 
.  liquely  on  the  mirror,  for  it  must  ascend  in  order  to  reach  it;  it 
will,  therefore,  be  reflected  in  the  line  A  D:  and  since  we  view 
objects  in  the  direction  of  the  reflected  rays,  which  reach  the 
eye,  and  since  the  image  appears  at  the  same  distance,  behind  the 
mirror,  that  the  object  is  before  it,  we  must  continue  the  line 
A  D  to  E,  and  the  line  C  D  to  F,  at  the  termination  of  which, 
the  image  will  be  represented. 

17.  By  what  experiment  can  you  prove  that  a  separate  image  of  an  object 
is  formed  in  each  eye?  18.  Under  what  circumstances  are  objects  seen  dou- 
ble ?  19.  Why  is  not  tha  image  of  an  object  inverted  in  the  common  mirror : 
20.  Your  whole  figure  -may  be  seen  in  a  looking-glass,  which  is  not  more 
than  half  your  height;  how  is  this  shown  in  fig.  3.  plate  17? 


REFLECTION    OF    MIRRORS. 


17i 


Emily,  Then  I  do  not  understand  why  I  should  not  see  the 
wliole  of  my  person  in  a  much  smaller  mirror,  for  a  ray  of  light 
fiom  my  feet  would  always  reach  it,  though  more  obliquely. 

Mrs.  B.  True;  but  the  more  obliquely  the  ray  falls  on  the 
mirror,  the  more  obliquely  it  will  be  reflected;  the  ray  would, 
therefore,  be  reflected  above  your  head,  and  you  could  not  see 
it.     This  is  shown  by  the  dotted  line  (fig.  3.) 

Now  stand  a  little"^  to  the  right  of  the  mirror,  so  that  the  rays 
of  light  from  your  figure  may  fall  obliquely  on  it 

Emily.     There  is  no  image  formed  of  me  in  the  glass  now.' 

Mrs.  B.  I  beg  your  pardon,  there  is;  but  you  cannot  see  it, 
because  the  incident  rays,  falling  obliquely  on  the  mirror,  will 
be  reflected  obliquely,  in  the  opposite  direction;  the  angles  of 
incidence,  and  reflection,  being  equal.  Caroline,  place  your- 
self in  the  direction  of  the  reflected  rays,  and  tell  me  whether 
you  do  not  see  Emily's  image  in  the  glass.** 

Caroline.  Let  me  consider. — In  order  to  look  in  the  direc*' 
tion  of  the  reflected  rays,  I  must  place  myself  as  much  to  the 
left  of  the  glass,  as  Emily  stands  to  the  right  of  it. — Now  I  see 
her  image,  not  straight  before  me,  however,  but  before  her;  and 
it  appears  at  the  same  distance  behind  the  glass,  that  she  is  in 
front  of  it. 

Mrs.  B.  You  must  recollect,  that  we  always  see  objects  in 
the  direction  of  tlie  last  rays,  which  reach  our  eyes.  Figure  4 
represents  an  eye,  looking  at  the  image  of  a  vase,  reflected  by 
a  mirror;  it  must  see  it  in  the  direction  of  the  ray  A  B,  as  that 
is  the  ray  which  brings  the  image  to  the  eye;  prolong  the  ray  to 
C,  and  in  that  spot  will  the  image  appear. 

Caroline.  I  do  not  understand  why  a  looking-glass  reflects 
the  rays  of  light;  for  glass  is  a  transparent  body,  which  should 
transmit  them  I 

Mrs.  B.  It  is  not  the  glass  that  reflects  the  rays  which  form 
the  image  you  beliold,  but  the  silvering  behind  it;  this  silvering 
is  a  compound  of  mercury  and  tin,  which  forms  a  brilliant  me- 
tallic coating.  The  glass  acts  chiefly  as  a  transparent  case, 
through  whicli  the  rays  find  an  easy  passage,  to,  and  from,  the 
quicksilver. 

Caroline.  Why  then  should  not  mirrors  be  made  simply  of 
mercury? 

Mrs.  B.  Because  mercury  is  a  fluid.  By  amalgamating  it 
with  tinfoil,  it  becomes  of  the  consistence  of  paste,  attaches 
itself  to  the  glass,  and  forms,  in  fact,  a  metallic  mirror,  which 

21.  Why  is  the  image  invisible  to  the  person,  when  not  standing  directly 
before  the  glass  ?  22.  In  what  situation  may  a  second  person  see  the  image 
reflected?  23.  In  what  direction  will  an  object  always  appear  to  the  eye? 
24.  How  is  this  explained  by  fig.  4,  plate  17  i*  25.  What  is  it  that  reflects 
the  rays  in  a  looking-glass  ? 


174  REFLECTION  OF  MIRRORS. 

would  be  much  more  perfect  without  its  glass  cover,  for  the 
purest  glass  is  never  perfectly  transparent;  some  of  the  yrjs, 
therefore,  are  lost  during  their  passage  through  it,  by  being 
either  absorbed,  or  irregularly  reflected. 

This  imperfection  of  glass  mirrors,  has  introduced  the  use  of 
metallic  mirrors,  for  optical  purposes. 

Emily.  But  since  all  opaque  bodies  reflect  the  rays  of  light, 
I  do  not  understand  why  they  are  not  all  mirrors. 

Caroline.  A  curious  idea  indeed,  sister;  it  would  be  very 
gratifying  to  see  oneself  in  every  object  at  which  one  looked. 

Mrs.  B.  It  is  very  true  that  all  o])aque  objects  reflect  light; 
but  the  surface  of  bodies,  in  general,  is  so  rough  and  uneven, 
that  the  reflection  from  them  is  extremely  irregular,  and  prevents 
the  rays  from  forming  an  ima^e  on  the  retina.  This,  you  will 
be  able  to  understand  better,  when  I  shall  explain  to  you  the  na- 
ture of  vision,  and  the  structure  of  the  eye. 

You  may  easilv  conceive  the  variety  of  directions  in  which 
rays  would  be  reflected  by  a  nutmeg-grater,  on  account  of  the 
inequality  of  its  surface,  and  the  number  of  holes  w  ith  which  it 
is  pierced.  All  solid  bodies  more  or  less  resemble  the  nutmeg- 
grater,  in  these  respects;  and  it  is  only  those  which  are  suscepti- 
ble of  receiving  a  polish,  tliat  can  be  made  to  reflect  the  rays 
with  regularity.  As  hard  bodies  are  of  the  closest  texture,  the 
least  porous,  and  capable  of  taking  the  highest  polish,  they  make 
the  best  mirrors;  none,  therefore,  are  so  well  calculated  for  this 
purpose,  as  metals. 

Caroline.  But  the  property  of  regular  reflection,  is  not  con- 
fined to  this  class  of  bodies;  for  I  have  often  seen  myself,  in  a 
liighly  polished  mahogany  table. 

Mrs.  B.  Certainly;  but  as  that  substance  is  less  durable, 
and  its  reflection  less  perfect,  than  that  of  metals,  I  believe  it 
would  seldom  be  chosen,  for  the  purpose  of  a  mirror. 

There  are  three  kinds  of  mirrors  used  in  optics;  the  plain^  or 
flat-,  which  are  the  common  mirroi's  we  have  just  mentioned; 
convex  mirrors,  and  concave  mirrors.  The  reflection  of  the  two 
latter,  is  very  diiFerent  from  that  of  the  former.  The  plain  mir- 
ror, we  have  seen,  does  not  alter  the  direction  of  the  reflected 
rays,  and  forms  an  image  behind  the  glass,  exactly  similar  to 
the  object  before  it.  A  convex  mirror  has  the  peculiar  proper- 
ty of  making  the  reflected  rays  diverge,  by  which  means  it 
diminishes  the  image;  and  a  concave  mirror  makes  the  rays 
converge,  and  under  certain  circumstances,  magnifies  the  image. 

Emily.     We   have  a  convex  mirror  in  the  drawing-room, 

26.  All  opaque  bodies  reflect  some  light,  why  do  they  not  all  act  as  mir- 
rors ?  27.  What  substances  form  the  most  perfect  mirrors,  and  for  what  rea- 
son ?  28.  What  are  the  three  kinds  of  mirrors  usually  employed  for  optical 
purposes  ?    20.  How  are  the  rays  of  light  affected  by  them  ? 


1*LATE 


REFLECTION  OF  MIRRORS.  175 

which  forms  a  beautiful  mininture  picture  of  tlie  objects  in  the 
room;  and  I  have  often  amused  myself  witli  looking  at  my 
magnified  face  in  a  concave  mirror.  Kut  I  hope  you  will  ex- 
plain to  us,  why  the  one  enlarges,  while  the  other  diminishes 
the  objects  it  reflects. 

Mrs.  B.  Let  us  begin  by  examining  tlie  reflection  of  a  con- 
vex mirror.  Tliis  is  formed  of  a  portion  of  the  exterior  surface 
of  a  sphere.  When  several  parallel  rays  fall  upon  it,  that  ray 
only  which,  if  prolonged,  would  pass  through  the  centre  or  axis 
t)f  the  mirror,  is  perpendicular  to  it.  In  order  to  avoid  confu- 
sion, I  have,  in  fig.  1,  plate  18,  drawn  only  three  parallel  lines, 
A  B,  C  I),  E  F,  to  represent  rays  falling  on  the  convex  mirror, 
M  N;  the  middle  ray,  you  will  observe,  is  perpendicular  to  the 
mirror,  the  others  fall  on  it,  obliquely. 

Caroline.  As  the  three  rays  are  parallel,  why  are  they  not  all 
perpendicular  to  the  mirror.^ 

Mrs.  B.  They  \Vould  be  so  to  a  flat  mirror;  but  as  this  is 
spherical,  no  ray  can  fall  perpendicularly  upon  it  which  is  not 
directed  towards  the  centre  of  the  sphere. 

Emily.  Just  as  a  weight  falls  perpendicularly  to  the  earth, 
when  gravity  attracts  it  towards  the  centre. 

Mrs.  B.  In  order,  therefore,  that  rays  may  fall  perpendicu- 
larly to  the  mirror  at  B  and  F,  the  rays  must  be  in  tlie  direction 
of  the  dotted  lines,  which,  you  may  observe,  meet  at  the  centre 
O  of  the  sphere,  of  which  the  mirror  forms  a  portion. 

Now,  can  you  tell  me  in  what  direction  the  three  rays,  A  B,  C 
D,  E  F,  wiirbe  reflected? 

Emilij.  Yes,  I  think  so:  the  middle  ray,  falling  perpendicu- 
larly on  the  mirror,  will  be  reflected  in  the  same  line:  the  two" 
outer  rays  falling  obliquely,  will  be  reflected  obliquely  to  Gand 
H;  for  the  dotted  lines  you  have  drawn  are  perpendiculars,  which 
divide  the  angles  of  incidence  and  reflection,  of  those  two 
rays. 

Mrs.  B.  Extremely  well>  Emily:  and  since  we  see  objects 
in  the  direction  of  tlie  reflected  ray,  we  shall  see  the  ima^^e  L, 
which  is  the  point  at  which  the  reflected  rays,  if  continued 
through  the  mirror,  would  unite  and  form  an  image.  This  point 
is  equally  distant,  from  tlie  surface  and  centre  of  the  sphere,  and 
is  called  the  imaginary  focus  of  the  mirror. 

Caroline.     Pray,  what  is  the  meaning  of  focus? 

Mrs.  B.     A  point  at  which  converging  rays,  unite.    And  it  is 

30.  What  is  the  form  of  a  convex  mirror,  and  how  do  parallel  rays  fall  upon 
it,  as  represented  in  fig.  1,  plate  18  ?  31.  What  is  represented  by  the  dotted 
line  in  the  same  figure  ?  32.  Explain  by  the  figure,  how  the  parallel  rays  will 
be  reflected.  33.  At  what  distance  behind  such  a  mirror,  would  an  image, 
produced  by  parallel  rays,  be  formed?  34.  What  is  that  point  d^omi- 
nated  ? 


176  REFLECTION  OF  CONVEX  MIRRORS. 

in  this  case,  called  an  imaginary  focus^  because  the  rays-  do  not 
really  unite  at  that  point,  but  only  appear  to  do  so:  for  the  rays 
do  not  pass  through  the  mirror,  since  they  are  reflected  by  it. 

Emily.  I  do  not  yet  understand  why  an  object  appears 
smaller,  when  viewed  in  a  convex  mirror. 

Mrs.  B.  It  is  owing  to  the  divergence  of  i\\Q^  reflected  rays. 
You  have  seen  that  a  convex  mirror,  by  reflection,  converts 
parallel  rays  into  divergent  rays;  rays  that  fall  upon  the  mirror 
divergent,  are  rendered  still  more  so  by  reflection,  and  conver- 
gent rays  are  reflected  either  parallel^  or  less  convergent.  If 
then,  an  object  be  placed  before  any  part  of  a  convex  mirror,  as 
the  vase  A  B,  fig.  !2,  for  instance,  the  two  rays  from  its  extremi- 
ties, falling  convergent  on  the  mirror,  will  be  reflected  less  con- 
vergent, and  will  not  come  to  a  focus,  till  they  arrive  at  C;  then 
an  eye  placed  in  the  direction  of  the  reflected  rays,  will  see  the 
image  formed  in  (or  rather  behind)  the  mirror,  at  a  h. 

Caroline.  But  the  reflected  rays,  do  not  appear  to  me  to  con- 
verge less  than  the  incident  rays.  1  should  have  supposed  that, 
on  the  contrary,  they  converged  more,  since  they  meet  in  a 
point. 

Mrs.  B.  They  would  unite  sooner  than  they  actually  do,  if 
tliey  were  not  less  convergent  than  the  incident  rays:  for  ob- 
serve, that  if  the  incident  rays,  instead  of  being  reflected  by  the 
mirror,  continued  their  course  in  their  original  direction,  they 
would  come  to  a  focus  at  D,  which  is  considerably  nearer  to  the 
mirror  than  at  C;  the  image,  is,  therefore,  seen  under  a  smaller 
angle  than  the  object;  and  the  more  distant  the  latter  is  from 
the  mirror,  tlie  smaller  is  the  image  reflected  by  it. 

You  will  now  easily  understand  the  nature  of  the  reflection  of 
concave  mirrors.  These  are  formed  of  a  portion  of  the  internal 
surface  of  a  hollow  sphere,  and  their  peculiar  property  is  to  con- 
verge the  rays  of  light. 

Can  you  discover,  Caroline,  in  what  direction  the  three 
parallel  rays,  X  B,  C  D,  E  F,  are  reflected,  which  fall  on  the 
concave  mirror,  M  N,  (fig.  3.)  ? 

Caroline.  I  believe  I  can.  The  middle  ray  is  sent  back  in 
the  same  line,  in  which  it  arrives,  that  being  the  direction  of  the 
axis  of  the  mirror;  and  the  two  others  will  be  reflected  obliquely, 
as  they  fall  obliquely  on  the  mirror.  I  must  now  draw  two 
dotted  lines  perpendicular  to  their  points  of  incidence,  which 
will   divide  their  angles  of  incidence  and  reflection;    and  in 

35.  What  is  meant  by  a  focus  ?  36.  Why  is  the  point  behind  the  mirror, 
called  the  imaginary  focus  ?  37.  Why  does  an  object  appear  to  be  lessened 
by  a  conyex  mirror,  (fig.  2.)  ?  38.  What  is  a  concave  mirror,  and  what  its 
peculiar  property  ?  39.  How  are  parallel  rays  reflected  by  a  coueave  mirror, 
as  explained  by  fig.  3,  plate  18.* 


aEFLECTION  OF  CONCAVE  MIRRORS.  177 

order  that  those  angles  may  be  equal,  the  two  oblique  rays  must 
be  reflected  to  L,  where  they  will  unite  with  the  middle  ray. 

Mrs.  B.  Very  well  explained.  Thus  you  see,  that  when 
any  number  of  parallel  rays  fall  on  a  concave  mirror,  they  are 
all  reflected  to  a  focus:  for  in  proportion  as  the  rays  are  more 
distant  from  the  axis  of  tiie  mirror,  they  fall  more  obliquely  upon 
it,  and  are  more  obliquely  reflected;  in  consequence  of  which 
they  come  to  a  focus  in  the  direction  of  the  axis  of  the  mirror,  at 
a  point  equally  distant  from  the  centre,  and  the  surface,  of  the 
sfMiere;  and  this  point  is  not  an  imaginary  focus,  as  happens 
with  the  convex  mirror,  but  is  the  true  focus  at  which  the  rays 
unite. 

Emily.  Can  a  mirror  form  more  than  one  focus,  by  reflecting 
rays.^ 

Mrs.  B.  Yes.  If  rays  fall  convergent  on  a  concave  mirror, 
(fig.  4.)  they  are  sooner  brought  to  a  focus,  L,  than  parallel  rays; 
their  focus  is,  therefore,  nearer  to  the  mirror  M  N.  Divergent 
rays  are  brought  to  a  more  distant  focus  than  parallel  rays,  as  in 
figure  5,  where  the  focus  is  at  L;  but  what  is  called  the  true 
focus  of  mirrors,  either  convex  or  concave,  is  that  of  parallel 
rays,  and  is  equally  distant  from  the  centre,  and  the  surface 
of  the  spherical  mirror. 

I  shall  now  show  you  the  real  reflection  of  rays  of  light,  bj^  a 
metallic  concave  mirror.  This  is  one  made  of  polished  tin, 
which  I  expose  to  the  sun,  and  as  it  shines  bright,  we  shall  be 
able  to  collect  the  rays  into  a  very  brilliant  focus.  1  hold  a 
piece  of  paper  where  I  imagine  the  focus  to  be  situated;  you 
may  see  by  the  vivid  spot  of  light  on  the  paper,  how  much  the 
rays  converge:  bu<>  it  is  not  yet  exactly  in  the  focus;  as  I  ap- 
proach the  paper  to  that  point,  observe  how  tlie  brightness  of  the 
spot  of  light  increases,  while  its  size  diminishes. 

Caroline.  That  must  be  occasioned  by  the  rays  approaching 
closer  together.  I  think  you  hold  the  paper  just  in  the  focus 
now,  the  li^ht  is  so  small  and  dazzling—Oh,  Mrs.  B.,  the  paper 
has  taken  hre! 

Mrs.  B.  The  rays  of  light  cannot  be  concentrated,  without, 
at  the  same  time,  accumulating  a  proportional  quantity  of  heat: 
hence  concave  mirrors  have  obtained  the  name  of  burning  mir- 
rors. 

Emily.  I  have  often  heard  of  the  surprising  effects  of  burn- 
ing mirrors,  and  I  am  quite  delighted  to  understand  their  na- 
ture. 

Caroline.     It  cannot  be  the  true  focus  of  the  mirror,  at  which 

40.  Where  is  the  focus  of  parallel  rays,  in  a  concave  mirror?  41.  If  rays 
fall  on  it  convergent,  how  are  they  reflected ?  42.  How  if  divergent  ?  43. 
How,  and  why,  may  concave,  become  burning  mirrors  ? 


178  THE  REFLECTION  OF  MIRRORS. 

the  rays  of  the  sun  unite,  for  as  they  proceed  from  so  large  a 
body,  they  cannot  fall  upon  the  mirror  parallel  to  each  other. 

Mrs,  B.  Strictly  speaking,  they  certainly  do  not.  But  when 
rays,  come  from  such  an  immense  distance  as  the  sun,  they  may 
be  considered  as  parallel:  their  point  of  union  is,  therefore,  the 
true  focus  of  the  mirror,  and  there  the  image  of  the  object  is  re- 
presented. 

Now  that  I  have  removed  the  mirror  out  of  the  influence  of 
the  sun's  rays,  if  I  place  a  burning  taper  in  the  focus,  how  will 
its  light  be  reflected.^  (Fig.  6.) 

Caroline.     That,  I  confess,  I  cannot  say. 

Mrs.  B.  The  ray  which  falls  in  the  direction  of  the  axis  of 
the  mirror,  is  reflected  back  in  the  same  line;  but  let  us  draw 
two  other  rays  from  the  focus,  falling  on  the  mirror  at  B  and  F; 
the  dotted  lines  are  perpendicular  to  those  points,  and  the  two 
rays  will,  therefore,  be  reflected  to  A  and  E. 

Caroline.  Oh,  now  I  understand  it  clearly.  The  rays  which 
proceed  from  a  light  placed  in  the  focus  of  a  concave  mirror  fall 
divergent  upon  it,  and  are  reflected,  parallel.  It  is  exactly  the 
reverse  of  the  former  experiment,  in  which  the  sun's  rays  fell 
parallel  on  the  mirror,  and  were  reflected  to  a  focus. 

Mrs.  B.  Yes:  when  the  incident  rays  are  parallel,  the  re- 
flected rays  converge  to  a  focus;  when,  on  the  contrary,  the  in- 
cident rays  proceed  from  the  focus,  they  are  reflected  parallel. 
This  is  an  important  law  of  optics,  and  since  you  are  now  ac- 
quainted with  the  principles  on  which  it  is  founded,  I  hope  that 
you  will  not  forget  it. 

Caroline.  I  am  sure  that  we  shall  not.  But,  Mrs.  B.,  you 
said  that  the  image  was  formed  in  the  focus  of  a  concave  mirror; 
yet  I  have  frequently  seen  glass  concave  mirrors,  where  the  ob- 
ject has  been  represented  within  the  mirror,  in  the  same  manner 
as  in  a  convex  mirror. 

Mrs.  B.  That  is  the  case  only,  when  the  object  is  placed 
between  the  mirror  and  its  focus;  the  image  then  appears  magni- 
fied behind  the  mirror,  or,  as  you  would  say,  within  it. 

Caroline.  I  do  not  understand  why  the  image  should  be 
larger  than  the  object. 

Mrs.  B.  This  results  from  the  convergent  property  of  the 
concave  mirror.  If  an  object,  A  B,  (fio-.  7.)  be  placed  between 
the  mirror  and  its  focus,  the  rays  from  its  extremities  fall  diver- 
gent on  the  mirror,  and  on  being  reflected,  become  less  divergent, 
as  if  they  proceeded  from  C :  to  an  eye  placed  in  that  situation, 

44.  Why  may  rays  of  light  coming;  from  the  sun,  be  viewed  as  parallel  to 
each  other?  45.  If  a  luminous  body,  as  a  burning  taper,  be  placed  in  the 
focus  of  a  concave  mirror,  how  will  the  rays  from  it,  be  reflected  ?  (fig.  6.) 
46.  What  feet  is  explained  by  fig.  7,  plate  18  ? 


SBFimage  will 


ON  REFRACTION  AND  COLOURS.  179 


image  will  appear  magnified  behind  the  mirror  at  a  6,  since  it 
is  seen  under  a  larger  angle  than  the  object. 

You  now,  I  hope,  understand  the  reflection  of  light  by  opaque 
bodies.  At  our  next  meeting,  we  shall  enter  upon  another  pro- 
perty of  light,  no  less  interesting,  and  which  is  called  refraction. 


CONVERSATION  XVI. 


ON  REFRACTION  AND  COLOURS. 

TRANSMISSION  OF  LIGHT  BY  TRANSPARENT  BODIES. — REFRACTION. — RE- 
FRACTION BY  THE  ATMOSPHERE. — REFRACTION  BY  A  LENS. — REFRAC- 
TION BY  THE  PRISM. OF  COLOUR  FROM  THE  RAYS  OF  LIGHT. — OF  THE 

COLOURS  OF  BODIES. 


The  refraction  of  light  will  furnish  the  subject  of  to-day's 
lesson. 

Caroline,  That  is  a  property  of  which  I  have  not  the  faintest 
idea. 

3Irs,  B.  It  is  the  effect  which  transparent  mediums  produce 
on  light  in  its  passage  through  them.  Opaque  bodies,  you  know, 
reflect  the  rays,  and  transparent  bodies  transmit  them;  but  it  is 
found,  that  if  a  ray^  in  passing  from  one  medium,  into  another  oj 
different  density,  fall  obliquely,  it  is  turned  out  of  its  course. 
The  ray  of  light  is  then  said  to  be  refracted. 

Caroline.  It  must  then  be  acted  on  by  some  new  power, 
otherwise  it  would  not  deviate  from  its  first  direction. 

Mrs.  B.  The  power  which  causes  the  deviation  of  the  ray, 
appears  to  be  the  attraction  of  the  denser  medium.  Let  us  sup- 
pose the  two  mediums  to  be  air,  and  water;  if  a  ray  of  light 
J)asses  from  air,  into  water,  it  is  more  strongly  attracted  by  the 
atter,  on  account  of  its  superior  density. 

Emily,     In  what  direction  does  the  water  attract  the  ray  ? 

1.  What  is  meant  by  the  refraction  of  light  ?  2.  What  is  believed  to  be 
the  cause  of  refraction  ? 


180  THE    REFRACTION    OF    LIGHT. 

Mrs.  B.  The  ray  is  attracted  perpendicularly  towards  the 
water^  in  the  same  manner  in  which  bodies  are  acted  upon  by 

If  then  a  ray,  A  B,  (fig.  1,  plate  19.)  fall  perpendicularly  on 
water,  the  attraction  of  the  water  acts  in  the  same  direction  as 
the  course  of  the  ray:  it  will  not,  therefore,  cause  a  deviation, 
and  the  ray  will  proceed  straight  on,  to  E.  But  if  it  fall  oblique- 
ly, as  the  ray  C  B,  the  water  will  attract  it  out  of  its  course. 
Let  us  suppose  the  ray  to  have  approached  the  surface  of  a  den- 
ser medium,  and  that  it  there  begins  to  be  affected  by  its  attrac- 
tion; this  attraction,  if  not  counteracted  by  some  other  power, 
would  draw  it  perpendicularly  to  the  water,  at  B;  but  it  is  also 
impelled  by  its  projectile  force,  which  the  attraction  of  the  den- 
ser medium  cannot  overcome;  die  ray,  therefore,  acted  m  by 
both  these  powers,  moves  in  a  direction  between  them,  and  in- 
stead of  pursuing  its  original  course  to  D,  or  being  implicitly 
guided  by  the  water  to  E,  proceeds  towards  F,  so  that  the  ray 
appears  bent  or  broken. 

Caroline.  I  understand  that  very  well;  and  is  not  this  the 
reason  that  oars  appear  bent  in  the  water? 

Mrs.  B.  It  is  owing  to  the  refraction  of  the  rays,  reflected 
by  the  oar;  but  this  is  in  passing  from  a  dense,  to  a  rare  medium, 
for  you  know  that  the  rays,  by  means  of  which  you  see  the  oar, 
pass  from  water  into  air. 

Emily.  But  I  do  not  understand  why  refraction  takes  place, 
when  a  ray  passes  from  a  dense  into  a  rare  medium;  1  should 
suppose  that  it  would  be  less,  attracted  by  the  latter,  than  by 
the  former. 

Mrs.  B.  And  it  is  precisely  on  that  accoiint  that  the  ray  is 
refracted.  Let  the  upper  half  of  fig.  2,'i'epresent  glass,  and 
the  lower  half  water,  let  C  B  represent  a  ray,  passing  obliquely 
from  the  glass,  into  water:  glass,  being  the  denser  medium,  the 
ray  will  be  more  strongly  attracted  by  that  which  it  leaves  than 
by  that  which  it  enters.  The  attraction  of  the  glass  acts  in  the 
direction  A  B,  while  the  impulse  of  projection  would  carry  the 
ray  to  F;  it  moves,  therefore,  between  these  directions  towards  D. 

Emily.  So  that  a  contrary  refraction  takes  place,  when  a  ray 
passes  from  a  dense,  into  a  rare  medium. 

Mrs.  B.  The  rule  upon  this  subject  is  this;  when  a  ray  of 
Hghf  passes  from  a  rare  into  a  dense  medium,  it  is  refracted  to- 
wards the  perpendicular;  when  from  a  dense  into  a  rare  medium^ 
it  is  refracted  from  the  perpendicular.  By  the  perpendicular  is 
meant  a  line,  at  right  angle  with  the  refracting  surface.     This 

3.  How  is  a  ray  refracted  in  passing  obliquely  from  air  into  water  ? 
4.  How  is  this  refraction  explained  in  fig.  1,  plate  19?  5.  What  is  fig.  2  in- 
tended to  explain  ?  6.  What  is  the  rule  respecting  refraction,  by  different 
mediums  ^ 


VA 


Tlj\te  XIX, 


Jxc,  J. 


Fu,.2 


T  1) 


iy.4. 


♦ 


\ 

V 

THE    REFRACTION    OF    LIGHT.  181 

may  be  seen  in  fig.  1,  and  fig.  2,  where  the  lines  A  E,  are  the 
perpendiculars. 

Caroline,  But  does  not  the  attraction  of  the  denser  medium 
aftect  the  ray  before  it  touches  it  ? 

Mrs.  B.  The  distance  at  which  the  attraction  of  the  denser 
medium  acts  upon  a  ray,  is  so  small,  as  to  be  insensiblej  it  ap- 
pears, therefore,  to  be 'refracted  only  at  the  point  at  which  it 
passes  from  one  medium  into  the  other. 

Now  that  you  understand  the  principle  of  refraction,  I  will 
show  3^ou  the  real  refraction  of  a  ray  of  li^ht.  Do  you  see  the 
flower  painted  at  the  bottom  of  the  inside  of  this  tea-cup? 
(Fi^.  3.) 

Emily.  Yes. — But  now  you  have  moved  it  just  out  of  sight  j 
the  rim  of  the  cup  hides  it. 

Mrs.  B.  Do  not  stir.  I  will  fill  the  cup  with  water,  and 
you  will  see  the  flower  again. 

Emily.  I  do,  indeed!  Let  me  try  to  explain  this:  when  you 
drew  the  cup  from  me,  so  as  to  conceal  the  flower,  the  rays  re- 
flected by  it,  no  longer  met  my  eyes,  but  were  directed  above 
them;  but  now  that  you  have  filled  the  cup  with  water,  they  are 
refracted,  and  bent  downwards  when  passing  out  of  the  water, 
into  the  air,  so  as  again  to  enter  mj  eyes. 

Mrs.  B.  You  have  explained  it  perfectly:  fig.  3.  will  help 
to  imprint  it  on  your  memory.  You  must  observe  that  when  the 
flower  becomes  visible  by  the  refraction  of  the  ray,  you  do  not 
see  it  in  the  situation  which  it  really  occupies,  but  the  image  of 
the  flower  appears  higher  in  the  cup;  for  as  objects  always  ajp- 
pear  to  be  situated  in  the  direction  of  the  rays  which  enter  the 
eye,  the  flower  will  be  seen  at  B,  in  the  direction  of  the  refracted 
ray. 

Emily.  Then,  when  we  see  the  bottom  of  a  clear  stream  of 
water,  the  rays  which  it  reflects,  being  refracted  in  their  pass- 
age from  the  water  into  the  air,  will  make  the  bottom  appear 
higher  than  it  really  is. 

Mrs.  B.  And  the  water  will  consequently  appear  more  shal- 
low. Accidents  have  frequently  been  occasioned  by  this  cir- 
cumstance; and  boys,  who  are  in  the  habit  of  bathing,  should  be 
cautioned  not  to  trust  to  the  apparent  shallowness  of  water,  as 
it  will  always  prove  deeper  than  it  appears. 

The  refraction  of  light  prevents  our  seeing  the  heavenly  bodies 
in  their  real  situation:  the  light  they  send  to  us  being  refracted 
in  passing  into  the  atmosphere,  we  see  the  sun  and  stars  in  the 
direction  of  the  refracted  ray;  as  described  in  fig.  4,  plate  19., 

7.  What  is  meant  by  the  perpendicular?  8.  How  does  fig.  3,  plate  19, 
elucidate  the  law  of  refraction  ?  9.  What  will  be  the  effect  on  the  apparent 
situation  of  the  flower  ?  10.  What  effect  has  refraction  upon  the  apparent 
depth  of  a  stream  of  water  f 

Q 


182  THE    REFRACTION    OF    LIGHT. 

the  dotted  line  represents  the  extent  of  the  atmosphere,  above  a 
portion  of  the  earth,  E  B  E:  a  ray  of  light  coming  from  the  sun 
S,  falls  obliquely  on  it,  at  A,  and  is  refracted  to  B;  then,  since 
we  see  the  object  in  the  direction  of  the  refracted  ray,  a  specta- 
tor at  B,  will  see  an  image  of  the  sun  at  C,  instead  of  its  real 
situation,  at  S. 

Emily.  But  if  the  sun  were  immediately  over  our  heads, 
its  rays,  falling  perpendicularly  on  the  atmosphere,  would  not 
be  refracted,  and  we  should  then  see  the  real  sun,  in  its  true 
situation. 

Mrs.  B.  You  must  recollect  that  the  sun,  is  vertical  only 
to  the  inhabitants  of  the  torrid  zone;  its  rays,  therefore,  are  al- 
ways refracted,  in  this  latitude.  There  is  also  another  obstacle 
to  our  seeing  the  heavenly  bodies  in  their  real  situations:  light, 
though  it  moves  with  extreme  velocity,  is  about  eight  minutes 
and  a  quarter,  in  its  passage  from  the  sun  to  the  earth;  tlierefore, 
when  the  rays  reach  us,  the  sun  must  have  /{uitted  the  spot  he 
occupied  on  their  departure;  yet  we  see  him  in  the  direction  of 
those  rays,  and  consequently  in  a  situation  which  he  had  aban- 
doned eight  minutes  and  a  quarter,  before. 

Emily.  When  you  speak  of  the  sun's  motion,  you  mean,  I 
suppose,  his  apparent  motion,  produced  by  the  diurnal  motion 
of  the  earth? 

Mrs.  B.  Certainly;  the  effect  being  the  same,  whether  it  is 
our  earth,  or  the  heavenly  bodies,  which  move:  it  is  more  easy  to 
represent  things  as  they  appear  to  be,  than  as  they  really  are. 

Caroline.  During  the  morning,  then,  when  tlie  sun  is  rising 
towards  the  meridian,  we  must  (from  the  length  of  time  the  li^ht 
is  in  reaching  us)  see  an  image  of  tlie  sun  below  that  spot  which 
it  really  occupies. 

Emily.  But  the  refraction  of  the  atmosphere,  counteracting 
this  effect,  we  may,  perhaps,  between  the  two,  see  the  sun  in  its 
real  situation. 

Caroline.  And  in  the  afternoon,  w^hen  the  sun  is  sinking  in 
the  west,  refraction,  and  the  length  of  time  which  the  light  is  in 
reaching  the  earth,  will  conspire  to  render  the  image  of  the  sun, 
higher  man  it  really  is. 

Mrs.  B.  The  refraction  of  tlie  sun's  rays,  by  the  atmosphere, 
prolongs  our  days,  as  it  occasions  our  seeing  an  image  of  the  sun, 
both  before  he  rises,  and  after  he  sets;  when  below  our  horizon, 
he  still  shines  upon  the  atmosphere,  and  his  rays  are  thence 
refracted  to  the  earth:  so  likewise  we  see  an  image  of  the  sun, 

11.  How  does  the  atmosphere  refract  the  rays  of  the  sun,  as  represented, 
fig.  4?  12.  Why  have  we  the  rays  of  the  sun  always  refracted?  13.  What 
length  of  time  is  required  for  light  to  travel  from  the  sun,  to  the  earth  ? 
14.  What  effect  has  this  upon  his  apparent  place  ?  15.  How  is  the  length  of 
the  day  affected  by  refractiwi.^' 


THE    REFRACTION    OF    LIGHT.  183 

previously  to  his  rising,  the  rays  that  fall  upon  the  atmosphere 
being  refracted  to  the  earth. 

Caroline.  On  the  other  hand,  we  must  recollect  that  light  is 
eight  minutes  and  a  quarter  on  its  journey;  so  that,  by  the  time 
it  reaches  the  earth,  the  sun  may,  perhaps,  have  risen  above  the 
horizon. 

Emily.     Pray,  do  not  glass  windows,  refract  the  light? 

Mrs.  B.  They  do;  but  this  refraction  would  not  be  percep- 
tible, were  the  surfaces  of  the  glass,  perfectly  flat  and  parallel; 
because,  in  passing  through  a  pane  of  glass,  the  rays  sutfer  two 
refractions,  which,  being  in  contrary  directions,  produce  nearly 
the  same  eft'ect  as  if  no  refraction  had  taken  place. 

Emily.     I  do  not  understand  that. 

Mrs.  B.  Fi^.  5,  plate  19,  will  make  it  clear  to  you:  A  A 
represents  a  thick  pane  of  glass,  seen  edgeways.  When  the 
ray  B  approaches  the  glass,  at  C,  it  is  refracted  by  it;  and  in- 
stead of  continuing  its  course  in  the  same  direction,  as  the  dot- 
ted line  describes,  it  passes  through  the  pane,  to  D;  at  that  point 
returning  into  the  air,  it  is  again  refracted  by  the  glass,  but  in 
a  contrary  direction  to  the  first  refraction,  and  in  consequence 
proceeds  to  E.  Now  you  must  observe  that  the  ray  B  C  and 
the  ray  D  E  being  parallel,  the  light  does  not  appear  to  imVo 
suffered  any  refraction :  the  apparent,  differing  so  little  from  the 
true  place  of  any  object,  when  seen  through  glass  of  ordinary 
thickness. 

Emily.  So  that  the  effect  which  takes  place  on  the  ray  en- 
tering the  glass,  is  undone  on  its  quitting  it.  Or,  to  express 
myself  more  scientifically,  when  a  ray  of  light  passes  from  one 
medium  into  another,  and  through  that  into  the  first  again,  the 
two  refractions  being  equal,  and  in  opposite  directions,  no  sensi- 
ble effect  is  produced. 

Caroline,  I  think  the  effect  is  very  sensible,  for,  in  looking 
through  the  glass  of  the  window,  I  see  objects  very  much  dis- 
torted; articles  which  I  know  to  be  straight,  appear  bent  and 
broken,  and  sometimes  the  parts  seem  to  be  separated  to  a  dis- 
tance from  each  other. 

Mrs.  B.  That  is  because  common  window  glass  is  not  flat, 
its  whole  surface  being  uneven.  Rays  from  any  object,  falling 
upon  it  under  different  angles,  are,  consequently,  refracted  in 
various  ways,  and  thus  produce  the  distortion  you  have  observed. 

Emily.  Is  it  not  in  consequence  of  refraction,  that  the 
glasses  in  common  spectacles,  magnify  objects  seen  through  them? 

Mrs.  B,    Yes.     Glasses  of  this  description  are  called  lenses^ 

16.  How  are  rays  refracted,  which  fall  obliquely  upon  a  flat  pane  of  glass, 
(fig.  5,  plate  19  ?)  17.  What  is  the  reason  that  objects  are  distorted,  when 
seen  through  common  window  glass.' 


184  ON  REFRACTION  AND  COLOURS. 

of  these,  there  are  several  kinds,  the  names  of  which  it  will  be 
necessary  for  you  to  learn.  Every  lens  is  formed  of  glass,  ground 
so  as  to  form  a  segment  of  a  sphere,  on  one,  or  both  sides.  They 
are  all  represented  at  fig.  1,  plate  20.  The  most  common,  is  the 
double  convex  lens,  D.  This  is  thick  in  the  middle,  and  thin  at 
the  edges,  like  common  spectacles,  or  reading  glasses.  A  B,  is 
a  plano-convex  lens,  being  flat  on  one  side,  and  convex  on  the 
other.  E  is  a  double  concave^  being,  in  all  respects,  the  reverse 
of  D.  C  is  a  plano-concave,  flat  on  one  side,  and  concave  on  the 
other.  F  is  called  a  meniscus,  or  concavo-convex,  being  concave 
on  one,  and  convex  on  the  other  side.  A  line  passing  through 
the  centre  of  a  lens,  is  called  its  axis. 

Caroline.  I  should  like  to  understand  how  the  rays  of  light 
are  refracted,  by  means  of  a  lens. 

Mrs.  B.  When  parallel  rays  (fig.  6.)  fall  on  a  double  con- 
vex lens,  that  only,  which  falls  in  the  direction  of  the  axis  of  the 
lens,  is  perpendicular  to  the  surface^  the  other  rays,  falling 
obliquely,  are  refracted  towards  the  axis,  and  will  meet  at  a  point 
beyond  the  lens,  called  \i%  focus.  . 

Of  the  three  rays,  ABC,  which  fall  on  the  lens  D  E,  the 
rays  A  and  C  are  refracted  in  their  passage  through  it,  to  a,  and 
c;  and  on  quitting  the  lens,  they  undergo  a  second  refraction 
in  the  same  direction,  which  unites  them'with  the  ray  B,  at  the 
focus  F. 

Emily.  And  what  is  the  distance  of  the  focus,  from  the  sur- 
face of  the  lens? 

Mrs.  B.  The  focal  distance  depends  both  upon  the  form  of 
the  lens,  and  on  the  refracting  power  of  the  substance  of  which 
it  is  made;  in  a  glass  lens,  both  sides  of  which  are  equally  con- 
vex, the  focus  is  situated  nearly  at  the  centre  of  the  sphere,  of 
which  the  surface  of  the  lens  forms  a  portion;  it  is  at  the  dis- 
tance, therefore,  of  the  radius  of  the  sphere. 

The  property  of  those  lenses  which  have  a  convex  surface,  is 
to  collect  the  rays  of  light  to  a  focus;  and  of  those  which  have  a 
concave  surface,  on  the  contrary,  to  disperse  them.  For  the 
rays  A  and  C,  falling  on  the  concave  lens  X  Y,  (fig.  7,  plate  19.) 
instead  of  converging  towards  the  ray  B,  in  the  axis  of  the  lens, 
will  each  be  attracted  towards  the  thick  edges  of  the  lens,  both 
on  entering  and  quitting  it,  and  will,  thererore,  by  the  first  re- 
fraction, be  made  to  diverge  to  a,  c,  and  by  the  second,  to  d,  e. 

Caroline.     And  lenses  which  have  one  side  flat,  and  the  other 

18.  What  is  meant  by  a  lens  ?  19.  What  are  the  five  kinds  called,  repre- 
sented at  fig.  1 ,  plate  20  ?  20.  What  is  meant  by  the  axis  of  a  lens  ?  21 .  How 
are  parallel  rays,  refracted  by  the  double  convex  lens,  fig.  6,  plate  19? 
22.  What  is  meant  by  the  focus  of  a  lens  ?  23.  What  is  the  focal  distance  of 
parallel  rays,  from  a  double  convex  lens  ?  24.  How  are  the  rays  refracted 
by  a  coacave  lens,  fig.  7,  plate  19  r* 


Oir  REFRACTION    AND    COLOURS.  185 

convex,  or  concave,  as  A  and  B,  (fig.  1,  plate  20.)  are,  I  suppose, 
less  powerful  in  their  refractions? 

Mrs.  B.  Yes,*  the  focus  of  the  plano-convex,  is  at  the  dis- 
tance of  the  diameter  of  a  sphere,  of  which  the  convex  surface 
of  the  lens,  forms  a  portion;  as  represented  in  figure  2,  plate 
20.  The  three  parallel  rajs,  A  B  C,  are  brought  to  a  focus  by 
the  plano-convex  lens,  X  Y,at  F. 

Emily.  You  have  not  explained  to  us,  Mrs.  B.,  how  the  lens 
serves  to  magnify  objects. 

Mrs.  B.  By  turning  again  to  fig.  6,  plate  19.  you  will  readi- 
ly understand  this.  Let  A  C,  be  an  object  placed  before  the 
lens,  and  suppose  it  to  be  seen  by  an  eye  at  F;  the  ray  from  the 
point  A, will  be  seen  in  the  direction  F  G,  that  from  C,  in  the  direc- 
tion F  H;  the  visual  angle,  therefore,  will  be  greatly  increased, 
and  the  object  must  appear  larger,  in  proportion. 

I  must  now  explain  to  you  the  refraction  of  a  ray  of  light,  by 
a  triangular  piece  of  glass,  called  a  prism.     (Fig.  3.) 

Emilij.  The  three  sides  of  this  glass  are  fiat;  it  cannot,  there- 
fore, brmg  the  rays  to  a  focus;  nor  do  I  suppose  that  its  refrac- 
tion will  be  similar  to  that  of  a  flat  pane  of  glass,  because  it  has 
not  two  sides  parallel;  I  cannot,  therefore,  conjecture  what  effect 
the  refraction  by  a  prism,  can  produce. 

Mrs.  B.  The  refractions  of  the  ray,  both  on  entering  and  on 
quitting  the  prism,  are  in  the  same  direction,  (Fig.  3.)  On  en- 
tering the  prism  P,  the  ray  A  is  refracted  from  B  to  C,  and  on 
quittmg  it  from  C  to  D.  In  the  first  instance  it  is  refracted  to- 
wards, and  in  the  last,  from  the  perpendicular;  each  causing  it  to 
deviate  in  the  same  way,  from  its  original  course,  A  B. 

I  will  show  you  this  by  experiment;  but  for  this  purpose  it 
will  be  advisable  to  close  the  window-shutters,  and  admit, 
through  the  small  aperture,  a  ray  of  light,  which  I  shall  refract, 
by  means  of  this  prism. 

Caroline.  Oli,  what  beautiful  colours  are  represented  on  the 
opposite  wall!  There  are  all  the  colours  of  the  rainbow,  and  with 
a  brightness,  I  never  saw  equalled.     (Fig.  4,  plate  20.) 

Emily.  I  have  seen  an  effect,  in  some  respects  similar  to  this, 
produced  by  the  rays  of  the  sun  shining  upon  glass  lustres;  but 
now  is  it  possible  that  a  piece  of  white  glass  can  produce  such  a 
variety  of  brilliant  colours? 

Mrs.  B.  The  colours  are  not  formed  by  the  prism,  but  ex- 
isted in  the  ray  previously  to  its  refraction. 

25.  What  is  the  effect  of  one  plane  side  in  a  lens  ?  26.  How  is  the  focus 
of  the  plano-convex  lens  situated,  fig.  2,  plate  20  ?  27.  How  does  a  convex 
lens  magnify  objects,  fig.  6,  plate  19  ?  28.  What  is  the  article  denominated 
which  is  represented  at  fig.  3,  plate  20?  29.  How  will  a  ray  be  refracted, 
which  enters  on  one  side  of  the  prism,  in  the  direction  A  B .''  30.  VV'hat  e^ 
feet  is  produced  by  this  refraction,  as  represented  in  fie.  4,  plate  20 '' 

Q2 


186  ON    REFRACTION    AND    COI^OURS. 

Caroline,  Yet,  before  its  refraction,  it  appeared  perfectly 
•white. 

Mrs.  B.  The  white  rajs  of  the  sun,  are  composed  of  rays, 
which,  when  separated,  produce  all  these  colours,  although  whea 
blended  together,  they  appear  colourless  or  white. 

Sir  Isaac  Newton,  to  whom  we  are  indebted  for  the  most  im- 
portant discoveries  respecting  light  and  colours,  was  the  first 
who  divided  a  white  ray  of  light,  and  found  it  to  consist  of  an 
assemblage  of  coloured  rays,  which  formed  an  image  upon  the 
wall,  such  as  you  now  see  exhibited,  (fig.  4.)  in  which  are  dis- 
played the  following  series  of  colours:  red,  orange,  yellow,  green, 
blue,  indigo,  and  violet 

Emily.     But  how  does  a  prism  separate  these  coloured  rays? 

Mrs.  B.  By  refraction.  It  appears  that  the  coloured  rays 
have  diiferent  degrees  of  refrangibilit^;  in  passing  through  the 
prism,  therefore,  they  take  diiferent  directions  according  to  their 
susceptibility  of  refraction.  The  violet  rays  deviate  most  from 
tlieir  original  course;  they  appear  at  one  of  the  ends  of  the  spec- 
trum, A  B:  contiguous  to  the  violet,  are  the  blue  rays,  being 
those  which  have  somewhat  less  refrangibility;  then  follow,  in 
succession,  the  green,  yellow,  orange,  and  lastly,  the  red,  which 
are  the  least  refrangible  of  the  coloured  rays. 

Caroline.  I  cannot  conceive  how  these  colours,  mixed  to- 
gether, can  become  white? 

Mrs.  B.  That  I  cannot  pretend  to  explain;  but  it  is  a  fact, 
that  the  union  of  these  colours,  in  the  proportions  in  which  they 
appear  in  the  spectrum,  produce  in  us  the  idea  of  whiteness.  If 
you  paint  a  circular  piece  of  card,  in  compartments,  with  these 
seven  colours,  as  nearly  as  possible  in  the  proportion,  and  of  the 
shade  exhibited  in  the  spectrum,  and  whirl  it  rapidly  on  a  pin, 
it  will  appear  white;  as  the  velocity  of  the  motion,  wdl  have  the 
effect  of  blending  the  colours,  in  the  impression  which  they  make 
upon  the  eye. 

But  a  more  decisive  proof  of  the  composition  of  a  white  ray  is 
afforded,  by  reuniting  these  coloured  rays,  and  forming  with 
them,  a  ray  of  white  light. 

Caroline.  If  you  can  take  a  ray  of  white  light  to  pieces,  and 
put  it  together  again,  I  shall  be  quite  satisfied. 

Mrs.  B.  This  can  be  done  by  letting  the  coloured  rays, 
which  have  been  separated  by  a  prism,  fall  upon  a  lens,  which 
will  converge  them  to  a  focus;  and  if,  when  thus  reunited,  we 
find  that  they  appear  white  as  they  did  before  refraction,  I  hope 
you  wlW  be  convinced  that  the  white  rays,  are  a  compound 

''  31.  Of  what  are  the  rays  of  white  light  said  to  be  composed  ?  32.  What 
colours  are  produced?  33.  By  what  property,  in  light,  does  refiraction 
enable  w*  to  separate  these  different  rays  ? 


ON  REFRACTION  AND  COLOURS.  187 

of  the  several  coloured  rays.  The  prism  P,  you  see,  (fig.  5.) 
separates  a  ray  of  white  liglit,  into  seven  coloured  rays,  and  the 
lens  L  L  brings  them  to  a  focus  at  F,  where  they  again  appear 
white. 

Caroline.  You  succeed  to  perfection:  this  is  indeed  a  most 
interesting  and  conclusive  experiment. 

Emily.  Yet,  Mrs.  B.,  1  cannot  help  thinking,  that  there 
may,  perhaps,  be  but  three  distinct  colours  in  the  spectrum,  red, 
yellow,  and  blue;  and  that  the  four  otliers  may  consist  of  two 
of  these  colours  blended  together;  for,  in  painting,  we  find,  tliat 
by  mixing  red  and  yellow,  we  produce  orange;  with  different 
proportions  of  red  and  blue,  we  make  violet  or  any  shade  of  pur- 
ple; and  yellow,  and  blue,  form  green.  Now,  it  is  very  natural 
to  suppose,  that  the  refraction  of  a  prism,  may  not  be  so  perfect 
as  to  separate  the  coloured  rays  of  light  completely,  and  that 
those  which  are  contiguous,  in  order  of  refrangibility,  may  en- 
croach on  each  other,  and  by  mixing,  produce  the  intermediate 
colours,  orange,  green,  violet,  and  indigo. 

Mrs.  B.  Your  observation  is,  I  believe,  neither  quite  wrong, 
nor  quite  right.  Dr.  Wollaston,  who  has  performed  many  ex- 
periments on  the  refraction  of  light,  in  a  more  accurate  manner 
than  had  been  previously  done,  by  receiving  a  very  narrow  line 
of  light  on  a  prism,  found  that  it  formed  a  spectrum,  consisting 
of  rays  of  four  colours  only;  but  they  were  not  exactly  those  you 
have  named  as  primitive  colours,  for  they  consisted  of  red,  green, 
Ijlue,  and  violet.  A  very  narrow  line  of  yellow  Was  visible,  at 
the  limit  of  the  red  and  green,  which  Dr.  Wollaston  attributed 
to  the  overlapping  of  the  edges  of  the  red  and  green  light. 

Caroline.  But  red  and  green  mixed  together,  do  not  produce 
yellow? 

Mrs.  B.  Not  in  painting;  but  it  may  be  so  in  the  primitive 
rays  of  the  spectrum.  Dr.  Wollaston  observed,  that,  by  increas- 
ing the  breadth  of  the  aperture,  by  which  the  line  of  light  was 
admitted,  the  space  occupied  by  each  coloured  ray  in  the  spec- 
trum, was  augmented,  in  proportion  as  each  portion  encroached 
on  the  neighbouring  colour,  and  mixed  with  it;  so  that  the  in- 
tervention of  orange  and  yellow,  between  the  red  and  green,  is 
o^t'ing,  he  supposes,  to  the  mixture  of  these  two  colours;  and  the 
blue  is  blended  on  the  one  side  with  the  green,  and  on  the 
other  with  the  violet,  forming  the  spectrum,  as  it  was  originally 
observed  by  Sir  Isaac  Newton,  and  which  I  have  just  shown 
you. 

The  rainbow,  which  exhibits  a  series  of  colours,  so  analogous 

34.  What  experiment  may  be  performed  with  a  piece  of  card,  so  as  to 
exemplify  the  compound  nature  of  light  ?  35.  How  can  the  same  be  shown 
by  a  lens,  fig.  5.  plate  20.'  36.  Is  it  certakt  tixat  there  are  seven  primitive 
colours  in  the  spectrum.' 


188  ON  REFRACTION  AND  COLOURS. 

to  those  of  the  spectrum/is  formed  by  the  refraction  of  the  sun's 
rays,  in  their  passage  through  a  shower  of  rain;"  every  drop  of 
which  acts  as  a  prism,  in  separating  tlie  coloured  rays  as  they 
pass  through  it;  the  combined  etFect  of  innumerable  drops,  pro- 
duces the  bow;  which  jou  know  can  be  seen,  only  when  there 
are  both  rain,  and  sunshine. ) 

Emily.  Pray,  Mrs.  B.,  cannot  the  sun's  rays  be  collected  to 
a  focus  by  a  lens,  in  the  same  manner  as  they  are  by  a  concave 
mirror? 

Mrs.  B.  The  same  effect  in  concentrating  the  rays,  is  pro- 
duced by  the  refraction  with  a  lens,  as  by  the  reflection  from  a 
concave  mirror:  in  tlie  first,(the  rays  pass  through  the  glass  and 
converge  to  a  focus,  behind  it;  jn  the  latter,  they  are  reflected 
from  the  mirror,  and  brought  to  a  focus,  before  it.  (A  lens,  when 
used  for  the  purpose  of  collecting  the  sun's  rays,^is  called  a 
burning  glass.  I  have  before  explained  to  you,  the  manner  in 
which  a  convex  lens,  refracts  the  rays,  and  brings  them  to  a 
focus;  (fig.  6,  plate  19.)  as  these  rays  contain  both  light  and 
heat,  the  latter,  as  well  as  the  former,  is  refracted;  and  intense 
heat,  as  well  as  light,  will  be  found  in  the  focal  point.  The 
sun  now  shines  very  bright;  if  we  let  the  rays  fall  on  this  lens, 
you  will  perceive  the  focus. 

Emily.  Oh  yes:  the  point  of  union  of  the  rays,  is  very  lumi- 
nous. I  will  hold  a  piece  of  paper  in  the  focus,  and  see  if  it  will 
take  fire.  The  spot  of  light  is  extremely  brilliant,  but  the  paper 
does  not  burn? 

Mrs.  B.  Try  a  piece  of  brown  ^aper; — that,  you  see,  takes 
fire  almost  immediately. 

Caroline.  This  is  surprising;  for  the  light  appeared  to  shine 
more  intensely,  on  the  white,, than  on  the  brown  paper. 

Mrs.  B.  The  lens  collects  an  equal  number  of  rays  to  a 
focus,  whether  you  hold  the  white  or  the  brown  paper,  there;  but 
the  white  paper  appears  more  luminous  in  the  focus,  because 
most  of  the  rays,  instead  of  entering  into  the  paper,  are  reflected 
by  it;  and  this  is  the  reason  that  the  paper  does  not  readily  take 
fire:  whilst,  on  the  contrary,  the  brown  paper,  which  absorbs 
more  light  and  heat  than  it  reflects,  soon  becomes  heated  and 

Caroline.  This  is  extremely  curious;  but  why  should  brown 
paper,  absorb  more  rays,  than  white  paper? 

Mrs.  B.  I  am  far  from  being  able  to  give  a  satisfactory 
answer  to  that  question.  We  can  form  but  mere  conjecture  on 
this  point;  it  is  supposed  that  the  tendency  to  absorb,  or  reflect 

37.  How  is  the  rainbow  produced,  and  what  is  necessary  to  its  production? 
38.  How  are  the  solar  rays  aflfected  by  a  convex  lens?  39.  Why  is  such  a 
lens,  called  a  burning  glass  ?  40.  Why  are  bodies  of  a  dark  colour,  more 
readily  inflamed,  than  those  which  are  white  ^ 


ON  REFRACTION  AND  COLOURS.  189 

rays,  .^depends  on  the  arrangement  of  the  minute  particles  of  the 
body,  and  that  this  diversity  of  arrangement  renders  some  bodies 
susceptible  of  reflecting  one  coloured  ray,  and  absorbing  the 
others;  whilst  other  bodies,  have  a  tendency  to  reflect  all  the 
colours,  and  others  again,  to  absorb  them  alL.' 

Emily.  And  how  do  you  know  which  colours  bodies  have  a 
tendency  to  reflect;  or  which  to  absorb? 

Mrs.  B,  {  Because  a  body  always  appears  to  be  of  the  colour 
which  it  reflects;  for,  as  we  see  only  by  reflected  rays,  it  can 
appear  of  the  colour  of  those  rays,  only? 

Caroline.  But  we  see  all  bodies  of  their  own  natural  colour, 
Mrs.  B.;  the  grass  and  trees,  green;  the  sky,  blue;  the  flowers 
of  various  hues. 

Mrs.  B.  ('True;  but  why  is  the  grass  green?'— because  it  ab- 
sorbs all,  except  the  green  rays;  it  is,  therefore,  these  only  which 
the  gi-ass  and  trees  reflect  to  our  eyes,  and  this  makes  them 
appear  green.  The  flowers,  in  the  same  manner,  reflect  the 
various  colours  of  which  they  appear  to  us;  the  rose,  the  red 
rays;  the  violet,  the  blue;  the  jonquil,  the  yellow,  &c.\ 

Caroline.  But  these  are  the  permanent  colours  of  the  grass 
and  flowers,  whether  the  sun's  rays  sliine  on  them  or  not. 

Mrs.  B.  /Whenever  you  see  those  colours,  the  flowers  must 
be  illumined  by  some  light;  and  light,  from  whatever  source  it 
proceeds,  is  of  the  same  nature;  composed  of  the  various  coloured 
rays  which  paint  the  grass,  the  flowers,  and  every  coloured  ob- 
ject in  nature. 

Caroline.  But,  Mrs.  B.,  the  grass  is  green,  and  the  flowers 
are  coloured,  whether  in  the  dark,  or  exposed  to  the  light? 

Mrs.  B.     Why  should  you  think  so? 

Caroline.     It  cannot  be  otherwise. 
i     Mrs.  B.     A  most  philosophical  reason  indeed!     But,  as  I 
never  saw  them  in  the  dark,  you  will  allow  me  to  dissent  from 
your  opinion. 

Caroline.  What  colour  do  you  suppose  them  to  be,  then,  in 
the  dark? 

Mrs.  B.  None  at  all;  or  black,  which  is  the  same  thing.  You 
can  never  see  objects,  without  light.  White  light  is  compounded 
of  rays,  from  which  all  the  colours  in  nature  are  produced;  there, 
therefore,  can  be  no  colour  without  light;  and  though  a  substance 
is  black,  or  without  colour,  in  the  dark,  it  may  become  co 
loured,  as  soon  as  it  becomes  visible.  It  is  visible,  indeed,  only 
by  the  coloured  rays  which  it  reflects;  therefore,  we  can  see  it 
only  when  coloured. 

41.  What  is  believed  to  be  the  reason,  why  some  bodies  absorb  more  ray3 
than  others  ?  42.  What  determines  the  colour  of  any  particular  body  ?  43. 
What  exemplifications  are  given?  44.  By  what  reasoning  is  it  proved,  that 
bodies  do  not  retain  their  colours  in  the  dark  ? 


190  ON  REFRACTION  AND  COLOURS. 

Caroline,  All  you  say  seems  very  true,  and  I  know  not  what 
to  object  to  it;  yet  it  appears  at  the  same  time  incredible !  What, 
Mrs.  B.,  are  we  all  as  black  as  negroes  in  the  dark?  you  make 
me  shudder  at  the  thought. 

Mrs.  B.  Your  vanity  need  not  be  alarmed  at  the  idea,  as 
you  are  certain  of  never  being  seen,  in  that  state. 

Caroline.  That  is  some  consolation,  undoubtedly;  but  what 
a  melancholy  reflection  it  is,  that  all  nature  which  appears  so 
beautifully  diversified  with  colours,  is  really  one  uniform  mass 
of  blackness! 

Mrs.  B.  Is  nature  less  pleasing  for  being  coloured,  as  well 
as  illumined,  by  the  rays  of  light?  and  are  colours  less  beautiful, 
for  being  accidental,  rather  than  essential  properties  of  bo- 
dies ? 

Providence  seems  to  have  decorated  nature  with  the  enchant- 
ing diversity  of  colours,  which  we  so  much  admire,  for  the  sole 
purpose  of  beautifying  the  scene,  and  rendering  it  a  source  of 
sensible  aratification :  it  is  an  ornament  which  embellishes 
nature,  whenever  we  behold  her.  What  reason  is  there  to  re- 
gret, that  she  does  not  wear  it  when  she  is  invisible? 

Emily.  I  confess,  Mrs.  B.,  that  I  have  had  my  doubts,  as 
well  as  Caroline,  though  she  has  spared  me  the  pains  of  express- 
ing them:  but  I  have  just  thought  of  an  experiment,  which,  if  it 
succeed,  will,  I  am  sure,  satisfy  us  both.  It  is  certain,  that  we 
cannot  see  bodies  in  the  dark,  to  know  whether  they  have  then 
any  colour.  Buv  M^e  may  place  a  coloured  body  in  a  ray  of  light, 
which  has  been  refracted  by  a  prism;  and  if  your  theory  is  true, 
the  body,  of  whatever  colour  it  naturally  is,  must  appear  of  the 
colour  of  the  ray  in  which  it  is  placed;  for  since  it  receives  no 
other  coloured  rays,  it  can  reflect  no  others. 

Caroline.  Oh !  that  is  an  excellent  thought,  Emily;  will  you 
stand  the  test,  Mrs.  B.  ? 

Mrs.  B.  I  consent:  but  we  must  darken  the  room,  and  ad- 
mit only  the  ray  which  is  to  be  refracted;  otherwise,  the  white 
rays  will  be  reflected  on  the  body  under  trial,  from  various  parts 
of  the  room.  With  what  do  you  choose  to  make  the  experi- 
ment? 

Caroline.  This  rose:  look  at  it,  Mrs.  B.,  and  tell  me  whe- 
ther it  is  possible  to  deprive  it  of  its  beautiful  colour? 

Mrs.  B.  We  shall  see. — I  expose  it  first  to  the  red  rays, 
and  the  flower  appears  of  a  more  brilliant  hue;  but  observe  the 
green  leaves — 

Caroline.  They  appear  neither  red  nor  green;  but  of  a  dingy 
brown  with  a  reddish  glow? 

45.  What  proof  of  the  truth  of  this  theory  of  colours,  may  be  aflforded  by 
the  prism  ? 


ON  REFRACTION  AND  COLOURS.  191 

Mrs.  B.  They  cannot  appear  green,  because  they  have  no  green 
rays  to  reflect;  neither  are  they  red,  because  green  bodies  ab- 
sorb most  of  the  red  raysp(  But  though  bodies,  from  the  arrange- 
ment of  their  particles,  have  a  tendency  to  absorb  some  rays,  and 
reflect  others,  yet  it  is  not  natural  to  suppose,  that  bodies  are  so 
perfectly  uniform  in  their  arrangement,  as  to  reflect  only  pure 
rays  of  one  colour,  and  perfectly  to  absorb  the  others;  it  is  found, 
i)\i  the  contrary,  that  a  body  reflects,  in  great  abundance,  the 
rays  which  determine  its  colour,  and  the  others  in  a  greater  or 
less  de«;ree,  in  proportion  a,s  they  are  nearer  to  or  furtlier  from  its 
own  colour,  in  the  order  of  refrangibiiity.  The  green  leaves  of 
the  rose,  therefore,  will  reflect  a  few  of  the  red  rays,  which, 
blended  with  their  natural  blackness,  give  them  that  brown 
tinge:  if  they  reflected  none  of  the  red  rays,  they  would  appear 
perfectly  black.     Now  I  shall  hold  the  rose  in  the  blue  rays — 

Caroline.  Oh,  Emily,  Mrs.  B.  is  right  1  look  at  the  rose:  it  is 
no  longer  red,  but  of  a  dingy  blue  colour. 

Emily.  This  is  the  most  wonderful,  of  any  thing  we  have  yet 
learnt.  But,  Mrs.  B.,what  is  the  reason  that  the  green  leaves, 
are  of  a  brighter  blue  than  the  rose? 

Mrs.  B.  The  green  leaves  reflect  both  blue  and  yellow  rays, 
which  produce  a  green  colour.  They  are  now  in  a  coloured 
ray,  which  they  have  a  tendency  to  reflect;  they,  therefore,  re- 
flect more  of  the  blue  rays  than  the  rose,  (which  naturally  ab- 
sorbs that  colour,)  and  will,  of  course,  appear  of  a  brighter 
blue. 

Emily.  Yet,  in  passing  the  rose  through  the  diflferent  colours 
of  the  spectrum,  the  flower  takes  them  more  readily  than  the 
leaves. 

Mrs.  B.  Because  the  flower  is  of  a  paler  hue.  Bodies  which 
reflect  all  the  rays,  are  white;  those  which  absorb  them  all, 
are  black:  between  these  extremes,  bodies  appear  lighter  or 
darker,  in  proportion  to  the  quantity  of  rays  they  reflect  or  ab- 
sorb. This  rose  is  of  a  pale  red;  it  approaches  nearer  to  white 
than  to  black,  and  therefore,  reflects  rays,  more  abundantly  than  it 
absorbs  them. 

Emily.  But  if  a  rose  has  so  strong  a  tendency  to  reflect  rays, 
I  should  imagine  that  it  would  be  of  a  deep  red  colour. 

Mrs.  B.  I  mean  to  say,  that  it  has  a  general  tendency  to 
reflect  rays.  Pale  coloured  bodies,  reflect  all  the  coloured  rays 
to  a  certain  degree,  their  paleness,  being  an  approach  towards 
whiteness:   but  they  reflect   one   colour  more   than  the  rest: 

46.  Why  will  gieen  leaves,  when  exposed  to  the  red  ray,  appear  of  a  dingy 
brown  ?  47.  Bodies,  in  general,  when  placed  in  a  ray  differing  in  colour  irom 
their  own,  appear  of  a  mixed  hue,  what  causes  this  ?  48.  Why  will  bodies 
of  a  pale,  or  light  hue,  most  perfectly,  assume  the  different  colours  of  the 
spectrum  ? 


192  ON  REFRACTION  AND  COLOURS. 

this  predominates  over  the  white,  and  determines  the  colour  of 
the  body.  Since,  then,  bodies  of  a  pale  colour,  in  some  degree 
reflect  all  the  rajs  of  liglit,  in  passing  through  the  various  co- 
lours of  the  spectrum,  they  will  reflect  them  all,  with  tolerable 
brilliancy;  but  will  appear  most  vivid,  in  the  ray  of  their  natural 
colour.  The  green  leaves,  on  the  contrary,  are  of  a  dark  colour, 
bearing  a  stronger  resemblance  to  black,  than  to  white;  they^ 
have,  therefore,  a  greater  tendency  to  absorb,  than  to  reflect 
rays;  and  reflecting  very  few  of  any,  but  the  blue,  and  yellow 
rays,  they  will  appear  dingy,  in  passing  through  the  other  co- 
lours of  the  spectrum. 

Caroline.  They  must,  however,  reflect  great  quantities  of 
the  green  rays,  to  produce  so  deep  a  colour. 

Mrs.  B.  f  Deepness  or  darkness  of  colour,  proceeds  rather 
from  a  deficiency,  than  an  abundance  of  reflected  rays.^Remem- 
ber,  that  if  bodies  reflected  none  of  the  rays,  they  would  be 
black;  and  if  a  body  reflects  only  a  few  green  rays,  it  will  ap- 
pear of  a  dark  green;  it  is  the  brightness,  and  intensity  of  the 
colour,  which  show  that  a  great  quantity  of  rays  are  reflected. 

Emily.  A  white  body,  then,  which  reflects  all  the  rays,  will 
appear  equally  bright  in  all  the  colours  of  the  spectrum. 

3Irs.  B.  Certainly.  And  this  is  easily  proved  by  passing  a 
sheet  of  white  paper,  through  the  rays  of  the  spectrum. 
(  White,  you  perceive,  results  from  a  body  reflecting  all  the 
rays  which  fall  upon  it;  black,  is  produced,  when  they  are  all  ab- 
sorbed ;;and  colour,  arises  from  a  body  possessing  the  power  to 
decompose  the  solar  ray,  by  absorbing  some  parts,  and  reflect- 
ing others. 

Caroline.  What  is  the  reason  that  articles  which  are  blue, 
often  appear  green,  by  candle-light? 

Mrs.  B.  The  light  of  a  candle,  is  not  of  so  pure  a  white  as 
that  of  the  sun:  it  has  a  yellowish  tinge,  and  when  refracted  by 
the  prism,  the  yellow  rays  predominate;  and  blue  bodies  reflect 
some  of  the  yellow  rays,  from  their  being  next  to  the  blue,  in  the 
order  of  refrangibility;  the  superabundance  of  yellow  ra^s,  which 
is  supplied  by  the  candle,  gives  to  blue  bodies,  a  greenish  huep 

Caroline.  Candle-light  must  then  give  to  all  bodies^  a  yel- 
lowish tinge,  from  the  excess  of  yellow  rays;  and  yet  it  is  a 
common  remark,  that  people  of  a  sallow  complexion,  appear 
fairer,  or  whiter,  by  candle-light. 

Mrs.  B.  The  yellow  cast  of  their  complexion  is  not  so  strik- 
ing, when  every  surrounding  object  has  a  yellow  tinge. 

Emily.     Pray,  why  does  the  sun  appear  red,  through  a  fog? 

49.  Upon  what  property  in  a  body,  does  the  darkness  of  its  colour  depend? 
50.  Why  do  some  bodies  appear  -white,  others  black,  and  others  of  different 
colours  ?    51 .  From  what  cause  do  blue  articles  appear  green,  by  candle-light  ? 


ON  REFRACTION  AND  COLOURS.  193 

Mrs.  B.  (it  is  supposed  to  be  owing  to  the  rays,  which  are 
most  refrangible,  being  also  the  most  easily  reflected  :;in  pass- 
ing through  an  atmosphere,  loaded  with  moisture,  as  in  foggy 
weather,  and  also  in  the  morning  and  evening,  when  mists  pre- 
vail, the  vioUt^  indigOy  blue,  and  green  rays,  are  reflected  back 
by  the  particles  which  load  the  air;  whilst  the  yelloiv,  orange, 
and  red  rays,  being  less  susceptible  of  reflection,  pass  on,  and 
reach  the  eye.  ~j 

Caroline,     And,  pray,  why  is  the  sky  of  a  blue  colour? 

Mrs,  B,  You  should  rather  say,  the  atmosphere;  for  the  sky 
is  a  very  vague  term,  the  meaning  of  which,  it  would  be  diflTi- 
cult  to  define,  philosophically. 

Caroline.  But  the  colour  of  the  atmosphere  should  be  white^ 
since  all  the  rays  traverse  it,  in  their  passage  to  the  garth. 

Mrs,  B.  Do  not  forget  that  the  direct  rays  of  light  which 
pass  from  the  sun  to  the  earth,  do  not  meet  our  eyes,  excepting 
when  we  are  looking  at  that  luminary,  and  thus  intercept  them; 
in  which  case,  you  know,  that  the  sun  appears  white.  The 
atmosphere  is  a  transparent  medium,  through  whicli  the  sun's 
rays  pass  freely  to  the  earth;  (but  the  particles  of  which  it  is 
composed,  also  reflect  the  rays  of  lignt,  and  it  appears  tha\ 
they  possess  the  property  of  reflecting  the  blue  rays,  the  most 
copiously:  the  light,  therefore,  which  is  reflected  back  into  the 
atmosphere,  from  the  surface  of  the  earth,  falls  upon  these 
particles  of  air,  and  the  blue  rays  are  returned  by  reflection: 
this  reflection  is  performed  in  every  possible  direction;  so  that 
whenever  we  look  at  the  atmosphere,  some  of  these  rays  fall 
upon  our  eyes;  hence  we  see  the  air  of  a  blue  colour.  If  the 
atmosphere  did  not  reflect  any  rays,  though  the  objects,  on  the 
surface  of  the  earth,  would  be  illuminated,  (the  sky  would  ap- 
pear perfectly  black.) 

Caroline,  Oh,  how  melancholy  would  that  be;  and  how  per- 
nicious to  the  sight,  to  be  constantly  viewing  bright  objects 
against  a  black  sky.  But  what  is  the  reason  that  bodies  often 
change  their  colour;  as  leaves,  which  wither  in  autumn,  or  a  spot 
of  ink,  which  produces  an  iron-mould  on  linen? 

Mrs,  B,  It  arises  from  some  chemical  change,  which  takes 
place  in  the  arrangement  of  the  component  parts;  by  which  they 
lose  their  tendency  to  reflect  certain  colours,  and  acquire  the 
power  of  reflecting  others. )  A  withered  leaf  thus  no  longer  reflects 
the  blue  rays;  it  appears,  therefore,  yellow,  or  has  a  slight  ten- 
dency to  reflect  several  rays,  which  produce  a  dingy  brown  colour. 

52.  What  is  believed  to  be  the  cause,  of  the  red  appearance  of  the  sun, 
through  a  fog-,  or  misty  atmosphere  ?  53.  From  what  is  the  blue  colour  of 
the  iky,  thought  to  arise  ?  54.  What  would  be  the  colour  of  the  sky,  did  not 
the  atmosphere  retlect  light?  55  From  what  cause  do  some  bodies  change 
their  colour,  as  leaves  formerly  green,  become  brown,  and  ink,  veliow  f 

R 


194  ON  REFRACTION  AND  COLOURS. 

An  ink  spot  on  linen,  at  first  absorbs  all  the  rays;  but,  from 
the  action  of  soap,  or  of  some  other  agent,  it  undergoes  a  chemi- 
cal change,  and  the  spot  partially  regains  its  tendency  to  reflect 
colours,  but  with  a  preference  to  reflect  the  yellow  rays,  and 
such  is  the  colour  of  the  iron-mould. 

Emily.  Bodies,  then,  far  from  being  of  the  colour  which  they 
appear  to  possess,  are  of  that  colour  to  which  they  have  the 
greatest  aversion,  with  which  they  will  not  incorporate,  but  re- 
ject, and  drive  from  them. 

Mrs.  B.  It  certainly  is  so;  though  I  scarcely  dare  venture 
to  advance  such  an  opinion,  whilst  Caroline  is  contemplating  her 
beautiful  rose. 

Caroline.  My  poor  rose !  you  are  not  satisfied  with  depriving 
it  of  colour,  but  even  make  it  have  an  aversion  to  it;  and  I  am 
unable  to  contradict  you. 

Emily.  Since  dark  bodies,  absorb  more  solar  rays  than  light 
ones,  the  former  should  sooner  be  heated  if  exposed  to  the  sun? 

Mrs.  B.  And  they  are  found,  by  experience,  to  be  so.  Have 
you  never  observed  a  black  dress,  to  be  warmer  than  a  white  one? 

Emily.  Yes,  and  a  white  one  more  dazzling ri^the  black  is 
heated  by  absorbing  the  rays,  the  white  is  dazzling,  by  reflecting 
them.  J 

Caroline.  And  this  was  the  reason  that  the  brown  paper  was 
burnt  in  the  focus  of  the  lens,  whilst  the  white  paper  exhibited 
the  most  luminous  spot,  but  did  not  take  fire. 

Mrs.  B.  It  was  so.  It  is  now  full  time  to  conclude  our 
lesson.  At  our  next  meeting,  1  shall  give  you  a  description  of 
the  eye. 

56.  Why  is  a  black  dress,  warmer  in  the  sunshine,  than  a  white  one  of  the 
same  texture  ? 


CONVERSATION  XVH. 


ON  THE  STRUCTURE  OF  THE  EYE,  AND  OPTICAL 
INSTRUMENTS. 

DESCRIPTION  OF  THE  EYE. — OF  THE  IMAGE  ON  THE  RETINA. — REFRAC- 
TION BY  THE  HUMOURS  OF  THE  EYE. — OF  THE  USE  OF  SPECTACLES. — 
OF  THE  SINGLE  MICROSCOPE. — OF  THE  DOUBLE  MICROSCOPE. — OF  THE 
SOLAR  MICROSCOPE. — MAGIC  LANTHORN.— REFRACTING  TELESCOPE.  — 
REFLECTING  TELESCOPE, 

MRS.    B. 

The  body  of  the  eye,  is  of  a  spherical  form:  (fig.  1.  plate  21.) 
it  has  two  membranous  coats,  or  coverings^  the  external  one,  a 
a  a,  is  called  the  sclerotica,  this  is  commonly  known  under  the 
name  of  the  white  of  the  eye;  it  has  a  projection  in  that  part  of 
the  eye  which  is  exposed  to  view,  b  b,  which  is  called(the  trans- 
parent corneajbecause,  when  dried,  it  has  nearly  the  consistence 
of  very  fine  Korn,  and  is  sufficiently  transparent  for  the  light  to 
obtain  free  passage  through  it. 

The  second  membrane  which  lines  the  cornea,  and  envelops 
the  eye,  is  called  the  choroid  j  c  c  c;  tliis  has  an  opening  in  front, 
just  beneath  the  cornea,  which  forms  the  pupil,  or  sight  of  the 
eye,  (/  (/,  through  which  the  rays  of  light  pass  into  the  eye. 
The  pupil  is  surrounded  by  a  coloured  border  called  the  iris: 
e  e,  which,  by  its  muscular  motion,  always  preserves  the  pupil 
of  a  circular  form,  whether  it  is  expanded  in  the  dark,  or  con- 
tracted by  a  strong  light.  This  you  will  understand  better  by 
examining  fig.  2. 

Emily.  I  did  not  know  that  the  pupil  was  susceptible  of  va- 
rying its  dimensions. 

Mrs.  B.  The  construction  of  the  eye  is  so  admirable,  that  it 
is  capable  of  adapting  itself,  more  or  less,  to  the  circumstances 
in  wnich  it  is  placed.  In  a  faint  light,  the  pupil  dilates  so  as  to 
receive  an  additional  quantity  of  rays,  and  in  a  strong  light,  it 

1.  What  is  the  form  of  the  body  of  the  eye?  fig.  1,  plate  21.  2.  What  is 
its  external  coat  called?  3.  What  is  the  transparent  part  of  this  coat  denomi- 
nated ?  4.  What  is  the  second  coat  named  ?  5.  What  opening  is  there  in 
this?  6.  What  is  the  coloured  part  which  surrounds  the  pupil?  7.  The 
pupils  iilate  and  contract,  what  purpose  does  this  answer? 


196  OPTICS. 


contracte,  in  order  to  prevent  the  intensity  of  the  light  from  in- 
juring the  optic  nerve.,'  pbserve  Emily's  eyes,  as  she  sits  look- 
ing towards  the  windows:  the  pupils  appear  very  small,  and 
the  iris,  large.  Now,  Emily,  turn  from  the  light,  and  cover 
your  eyes  with  your  hand,  so  as  entirely  to  exclude  it,  for  a  ftw 
moments?) 

Caroline.  How  very  much  the  pupils  of  her  eyes  are  now 
enlarged,  and  the  iris  diminished!  This  is,  no  doubt,  the  reason 
why  the  eyes  suifer  pain,  when  from  darkness,  they  suddenly 
come  into  a  strong  light;  for  the  pupil  being  dilated,  a  quantity 
of  rays  must  rush  in,  before  it  has  time  to  contract. 

Emily.  And  when  we  go  from  a  strong  light,  into  obscurity, 
we  at  first  imagine  ourselves  in  total  darkness;  for  a  sufficient 
number  of  rays  cannot  gain  admittance  into  the  contracted 
pupil,  to  enable  us  to  distinguish  objects:  but  in  a  few  minutes  it 
dilates,  and  we  clearly  perceive  objects  which  were  before  in- 
visible. , 

Mrs.  B.  It  is  just  so.  The  choroid  c  c,  is  embued  with  a 
black  liquor,  which  serves  to  absorb  all  the  rays  that  are  irregu- 
larly reflected,  and  to  convert  the  body  of  the  eye,  into  a  more 
perfect  camera  obscura.  When  the  pupil  is  expanded  to  its  ut- 
most extent,  it  is  capable  of  admitting  ten  times  the  quantity  ot 
lio-ht,  that  it  does  when  most  contracted,  iln  cats,  and  animals 
wtiich  are  said  to«ee  in  the  dark,  the  power  of  dilatation  and  con- 
traction of  the  pupil,  is  still  greater;  it  is  computed  that  the 
pupils  of  Iheir  eyes  may  admit  one  hundred  times  more  light  at 
one  time  than  at  anotlier.  . 

Within  these  coverings  of  the  eye-ball,  are  contained,  three 
transparent  substances,  called  humours.  *  The  first  occupies  the 
space  immediately  behind  the  cornea,  and  is  called  the  aqueous 
humour,  //,  from  its  liquidity  and  its  resemblance  to  water.  Be- 
yond this,  is  situated  the  crystalline  humour,  g  g,  so  called  from 
its  clearness  and  transparency:  it  has  the  form  of  a  lens,  and 
refracts  the  rays  of  light  in  a  greater  degree  of  perfection,  than 
any  that  have  been  constructed  by  art:  it  is  attached  by  two 
muscles,  m  m,  to  each  side  of  the  choroid.  The  back  part  of  the 
eye,  between  the  crystalline  humour  and  the  retina,  is  tilled  by 
•  the  vitreous  humour,  h  h,  which  derives  its  name  from  a  resem- 
blance it  is  supposed  to  bear,  to  glass,  or  vitrified  substances. 

The  membranous  coverings  of  the  eye  are  intended  chielly  tor 
the  preservation  of  the  retina,  i  i,  which  is  by  far  the  most  im- 
portant part  of  the  eye,  as  it  is  that  which  receives  the  impres- 

8  How  could  you  observe  the  dilatation  and  contraction  of  the  pupils  ?  9. 
What  purpose  is  the  choroid  said  to  answer?  10.  In  what  animals  is  the 
chan-e  in  the  iris  greatest?  11.  What  are  th«  three  humours  denommated, 
and  how  are  they  situated? 

-if 


Plate  3X1. 


^'M 


OPTICS.  197 

sion  of  the  objects  of  sight,  and  conveys  it  to  the  mind.  The 
retina  is  formed  by  the  expansion  of  the  optic  nerve,  and  is  of  a 
most  perfect  whiteness  f  this  nerve  proceeds  from  the  brain,  en- 
ters the  eye,  at  zi,  on  the  side  next  the  nose,  and  is  finely  spread 
over  the  interior  surface  of  the  choroid. 

The  rays  of  light  which  enter  the  e^e,  by  the  pupil,  are  re- 
fracted by  the  several  humours  in  their  passage  through  them, 
and  unite  in  a  focus  on  the  retina.; 

Caroline.  I  do  not  understand  the  use  of  these  refracting 
humours:  the  image  of  objects  was  represented  in  the  camera 
obscura,  without  any  such  assistance. 

Mrs.  B.  That  is  true;  but  the  representation  became  much 
more  strong  and  distinct,  when  we  enlarged  the  opening  of  the 
camera  obscura,  and  received  the  rays  into  it,  through  a  lens. 

I  have  told  you,  that  rays  proceed  from  bodies  in  all  possible 
directions.  We  must,  therefore,  consider  every  part  of  an  ob- 
ject which  sends  rays  to  our  eyes,  as  points  from  which  the  rays 
diverge,  as  from  a  centre. 

Emily.  These  divergent  rays,  issuing  from  a  single  point,  I 
believe  you  told  us,  were  called  a  pencil  of  rays.^* 

Mrs.  B.  Yes.  Now,  divergent  rays,  on  entering  the  pupil, 
do  not  cross  each  other;  the  pupil,  however,  is  sufficiently  large  to 
admit  a  small  pencil  of  them;  and  these„if  not  refracted  to  a 
focus,  by  the  humours,  would  continue  diverging  after  they  had 
passed  the  pupil,  would  fall  dispersed  upon  the  retina,  and  thus 
the  image  of  a  single  point,  would  be  expanded  over  a  large  por- 
tion of  the  retina.  The  divergent  rays  from  every  other  point 
of  the  object,  would  be  spread  over  a  similar  extent  of  space, 
and  would  interfere  and -be  confounded  with  the  first;  so  that  no 
distinct  image  could  be  formed,  and  the  representation  on  the  re- 
tina would  be  confused,  both  in  figure  and  colour.'  Fig.  3.  repre- 
sents two  pencils  of  rays,  issuing  from  two  points  of  the  tree,  A  B, 
and  entering  the  pupil  C,  refracted  by  the  crystalline  humour  D, 
and  forming  on  tlie  retina,  at  a  b,  distinct  images  of  the  spot  they 
proceed  from.  Fi^.  4.  ditfers  from  the  preceding,  merely  from 
not  being  supplied  with  a  lens;  in  consequence  of  which,  the 
pencils  of  rays  are  not  refracted  to  a  focus,  and  no  distinct  im- 
age is  formed  on  the  retina.  I  have  delineated  only  the  rays 
issuing  from  two  points  of  an  object,  and  distinguished  the  two 
pencils  in  fig.  4.  by  describing  one  of  them  with  dotted  lines: 
the  interference  of  these  two  pencils  of  rays  on  the  retina,  will 
enable  you  to  form  an  idea  of  the  confusion  which  would  arke, 

12.  What  is  the  part  represented  at  i  i,  and  of  what  does  it  consist?  13. 
What  are  the  respective  uses  of  the  humours,  and  of  the  retina?  14 
Why  is  it  necessary  the  rays  should  be  refracted?  15.  How  is  this  illustrat 
ed  by  fig.  3  and  4,  plate  21  ? 

R  2 


198  OPTICS. 

from  thousands  and  millions  of  points,  at  tlie  same  instant  pour- 
ing their  divergent  rays  upon  the  retina. 

Emily.  True;  but  I  do  not  yet  well  understand,  how  the 
refracting  humours,  remedy  this  imperfection. 

Mrs.  B.  The  refraction  of  these  several  humours,  unites  the 
whole  of  a  pencil  of  rays,  proceeding  from  any  one  point  of  an 
object,  to  a  corresponding  point  on  the  retina,  and  the  image  is 
thus  rendered  distinct  and  strong.  If  you  conceive,  in  fig.  3., 
every  point  of  tlie  tree  to  send  fortli  a  pencil  of  rays,  similar  to 
those  from  A  B,  every  part  of  the  ti'ee  will  be  as  accurately 
represented  on  the  retina,  as  the  points  a  b. 

Emily.     How  admirably,  how  wonderfully,  is  this  contrived! 

Caroline.  But  since  the  eye  absolutely  requires  refracting 
humours,  in  order  to  have  a  distinct  representation  formed  on  the 
retina,  why  is  not  the  same  refraction  equally  necessary,  for  the 
images  formed  in  the  camera  obscura  ^ 

Mrs.  B.  It  is;  excepting  the  aperture  through  which  we  re- 
ceive the  rays  into  the  camera  obscura,  is  extremely  small;  so 
that  but  very  few  of  the  rays  diverging  from  a  point,  gain  afdmit- 
tance;  but  when  M'e  enlarged  the  aperture,  and  furnished  it  with 
a,  lens,  you  found  the  landscape  more  perfectly  represented. 

Caroline.  I  remember  how  obscure  and  confused  the  image 
was,  when  you  enlarged  the  opening,  without  putting  in  the  lens. 

Mrs.  B.  Such,  or  very  similar,  would  be  the  representation 
on  the  retina,  unassisted  by  the  refracting  humours. 

You  will  now  be  able  to  understand  the  nature  of  that  imper- 
fection of  sight,  which  arises  from  the  eyes  being  too  prominent. 
In  such  cases,  the  crystalline  humour,  D,  (fig.  5.)  being  extreme- 
ly convex,  refracts  the  rays  too  much,  and  collects  a  pencil, 
proceeding  from  the  object  A  B,  into  a  focus,  F,  before  they 
reach  the  retina.  From  this  focus,  the  rays  proceed,  diverging, 
and  consequently  form  a  very  confused  image  on  the  i-etina,  at 
a  b.     This  is  the  defect  in  short-sighted  people. 

Emily.  I  understand  it  perfectly.  But  why  is  this  defect 
remedied  by  bringing  the  object  nearer  to  the  eye,  as  we  find  to 
be  the  case  with  short-sighted  people? 

M?-s.  B.  The  nearer  you  bring  an  object  to  your  eye,  the 
more  divergent  the  rays  m\\  upon  the  crystalline  humour,  and 
consequently  they  are  not  so  soon  converged  to  a  focus:  this 
focus,  therefore,  either  falls  upon  the  retina,  or  at  least  ap- 
proaches nearer  to  it,  and  the  object  is  proportionably  distinct, 
as  in  fig.  6. 

Emily.  The  nearer,  then,  you  bring  an  object  to  a  lens,  the 
further  tlie  image  recedes  behind  it. 

16.  What  causes  a  person  to  be  short-sighted?  fig.  5,  plate  21.  17.  Why 
does  placing  an  object  near  the  eye,  enable  such,  to  see  distinctly  ?  fig.  6, 


'^f^^r' 


TLj\TE:xxnL 


OPTICS.  199 

Mrs.  B.  Certainly.  But  short-sighted  persons  have  another 
resource,  for  objects  which  they  can  not  bring  near  to  tlieir  eyes; 
this  is,  to  place  a  concave  lens,  C  D,  (fig.  1,  plate  22.)  before 
the  eye,  in  order  to  increase  ,the  divergence  of  the  rays.  The 
eftect  of  a  concave  lens,  is,  you  know,  exactly  the  reverse  of  a 
convex  one:  it  renders  parallel  rays  divergent,  and  those  which 
are  already  divergent,  still  more  so.  By  the  assistance  of  such 
glasses,  therefore,  the  rays  from  a  distant  object,  fall  on  the 
pupil,  as  divergent  as  those  from  a  less  distant  object;  and,  with 
short-sighted  people,  they  throw  the  image  of  a  distant  object, 
back,  as  far  as  the  retina. 

Caroline.     This  is  an  excellent  contrivance,  indeed. 

Mrs.  B.  And  tell  me,  what  remedy  w^ndd  you  devise  for 
such  persons  as  have  a  contrar}^  defect  in  their  sight;  tJiat  is  to 
say,  who  are  long-sighted,  in  whom  the  cnstalline  humour,  be- 
ing too  tlat,  does  not  refract  the  rays  sufficiently,  so  that  they 
leach  the  retina  before  they  are  converged  to  a  point? 

Caroline.  I  suppose  that  a  contrary  remedy  must  be  applied 
to  this  defect;  that  is  to  say,  a  convex  lens,  L  M,  fio-.  2,  to 
make  up  for  the  deficienfcy  of  convexity  of  the  crystalline  hu- 
mour, O  P.  For  the  convex  lens  would  bring  the  rays  nearer 
together,  so  that  they  would  fall,  either  less  divergent,  or  paral- 
lel, on  the  crystalline  humour;  and,  by  being  sooner  converged 
Jo  a  focus,  would  fall  on  the  retina. 

Mrs.  B.  Very  well,  Caroline.  This  is  the  reason  why 
elderly  people,  the  humours  of  whose  eyes  are  decayed  by  age, 
^re  under  the  necessity  of  usin^  convex  spectacles.  And  when 
deprived  of  that  resource,  they  hold  the  object  at  a  distance  from 
their  eyes,  as  in  fig  3,  in  order  to  bringc.the  focus  more  forward. 

Caroline.  I  have  often  been  surprised,  when  my  grandfather 
reads  without  his  spectacles,  to  see  him  hold  the  book  at  a  con- 
siderable distance  from  his  eyes.  But  I  now  understand  the 
cause;  the  more  distant  the  object  is  from  the  crystalline  lens, 
the  nearer  to  it,  will  the  image  be  formed. 

Emily.  I  comprehend  the  nature  of  these  two  opposite  de- 
fects very  well;  but  I  cannot  now  conceive,  how  any  sight  can 
be  perfect:  for,  if  the  crystalline  humour  is  of  a  proper  degree 
ofconvexity,  to  bring  the  image  of  distant  objects  to  a  focus  on 
the  retina,  it  will  not  represent  near  objects  distinctly;  and  if, 
on  the  contrary,  it  is  adapted  to  give  a  clear  image  of  near  ob- 
jects, it  will  produce  a  very  imperfect  one,  of  distant  objects. 

Mrs.  B,  Your  observation  is  very  good,  Emily;  and  it  is 
true,  that  every  person  would  be  subject  to  one  of  these  two 

18.  A  concave  lens  remedies  this  defect;  how?  fig.  1,  platt  22.  19.  What 
is  the  remedy,  when  a  person  is  long-sighted?  fig.  2.  20.  Why  Joes  holding 
an  object  far  from  the  eye,  help  such  persons  ?  fig.  3. 


SOO  OPTICS. 

defects,  if  we  had  it  not  in  our  power  to  adapt  the  eye,  to  the 
distance  of  the  object;  it  is  believed  that  this  is  accomplished, 
by  our  having  a  command  over  the  crystalline  lens,  so  as  to  pro- 
ject it  towards,  or  draw  it  back  from  the  object,  as  circum- 
stances require,  by  means  of  the  two  muscles,  to  which  the 
crystalline  humoUr  is  attached;  so  that  the  focus  of  the  rays, 
constantly  falls  on  the  retina,  and  an  image  is  formed  equally 
distinct,  either  of  distant  objects,  or  of  those  which  are  near. 

Caroline.  In  the  eyes  of  fishes,  which  are  the  only  eyes  I 
have  ever  seen  separate  from  the  head,  the  cornea  does  not  pro- 
trude, in  that  part  of  the  eye  wliich  is  exposed  to  view. 

3Irs.  B.  The  cornea  of  the  eye  of  a  fish  is  not  more  convex 
than  the  rest  of  the  ball  of  the  eye;  but  to  supply  this  deficiency, 
their  crystalline  humour  is  spherical,  and  refracts  the  rays  so 
much,  that  it  does  not  require  the  assistance  of  the  cornea  to 
bring  them  to  a  focus  on  the  retina. 

Emily.  Pray,  what  is  the  reason  that  we  cannot  see  an  ob- 
ject distinctly,  if  we  place  it  very  near  to  the  eye? 

Mrs.  B.  Because  the  rays  fall  on  the  crystalline  humour,  too 
divergent  to  be  refracted  to  a  focus  ort  the  retina;  the  confusion, 
therefore,  arising  from  viewing  an  object  too  near  the  eye,  is 
similar  to  that  which  proceeds  from  a  flattened  crystalline  hu- 
mour; the  rays  reach  the  retina  before  they  are  collected  to  a 
focus,  (fig.  4.)  If  it  were  not  for  this  imperfection,  we  should 
be  able  to  see  and  distinguish  the  parts  of  objects,  which,  from 
their  minuteness,  are  now  invisible  to  us;  for,  could  we  place 
them  very  near  the  eye,  the  image  on  the  retina  would  be  so 
much  magnified,  as  to  render  them  visible. 

Emily.  And  could  there  be  no  contrivance,  to  convey  the 
rays  of  objects  viewed,  close  to  the  eye,  so  that  they  should  be 
retracted  to  a  focus  on  the  retina? 

Mrs.  B.  The  microscope  is  constructed  for  this  purpose. 
The  single  microscope  (fig.  5.)  consists  simply  of  a  convex  lens, 
commonly  called  a  magnifying  glassy  in  the  focus  of  which  the 
object  is  placed,  and  through  which  it  is  viewed:  by  this  means, 
you  are  enabled  to  place  your  eye  very  near  to  the  object,  for 
'the  lens  A  B,  by  diminishing  the  divergence  of  the  rays,  before 
they  enter  the  pupil  C,  makes  them  fall  parallel  on  the  crystal- 
line humour  D,  by  which  they  are  refracted  to  a  focus  on  the 
retina,  at  R  R. 

Emily.  This  is  a  most  admirable  invention,  and  nothing  can 
be  more  simple;  for  the  lens  magnifiesthe  object,  merely  by  aU 
lowing  us  to  bring  it  nearer  to  the  eye. 

21.  How  is  the  eye  said  to  adapt  itself  to  distant,  and  to  near  objects? 
22.  Why  are  objects  rendered  indistinct,  when  placed  very  near  to  the  eye? 
fig.  4,  plate  22.  23.  What  is  the  single  microscope,  fig.  5,  and  how  does  it 
mi^gnify  objects  ? 


\  \ 


OPTICS.  201 

Mrs,  B.  Those  lenses,  therefore,  which  have  the  shortest 
focus  will  magnify  the  object  most,  because  they  enable  us  to 
place  it  nearest  to  the  eye. 

Emily.  But  a  lens,  that  has  the  shortest  focus,  is  most  bulg- 
ing or  convex;  and  the  protuberance  of  the  lens  will  prevent  the 
eye  from  approaching  very  near  to  the  object. 

3Irs.  B.  This  is  remedied  by  making  the  lens  extremely 
small:  it  may  then  be  spherical  without  occupying  much  space, 
and  thus  unite  the  advantages  of  a  short  focus,  and  of  allowing 
the  eye  to  approach  the  object. 

There  is  a  mode  of  magnifying  objects,  without  the  use  of  a 
lens:  if  you  look  through  a  hole,  not  larger  than  a  small  pin, 
you  may  place  a  minute  object  near  to  the  eye,  and  it  will  be 
distinct,  and  gi-eatly  enlarged.  This  piece  of  tin  lias  been  per- 
forated for  the  purpose;  place  it  close  to  your  eye,  and  this  small 
print  before  it. 

Caroline.  Astonishing!  the  letters  appear  ten  times  as  large 
as  they  do  without  it:  I  cannot  conceive  how  this  eftect  is  pro- 
duced. 

Mrs.  B.  The  smallness  of  the  hole,  prevents  the  entrance 
into  the  eye,  of  those  parts  of  every  pencil  of  rays  which  diverge 
much;  so  that,  notwithstanding  the  nearness  of  the  object,  those 
rays  from  it,  which  enter  the  eye,  are  nearly  parallel,  and  are, 
therefore,  brought  to  a  focus  by  the  humours  of  the  eye. 

Caroline.  We  have  a  microscope  at  home,  which  is  a  much 
more  complicated  instrument  than  that  you  have  described. 

Mrs.  B.  It  is  a  double  microscope,  (fig.  6.)  in  wliich  you 
see,  not  the  object  A  B,  but  a  magnified  image  of  it,  a  b.  In 
this  microscope,  two  lenses  are  employed;  the  one,  L  M,  for  the 
purpose  of  magnifying  the  object,  is  called  the  object-glass,  the 
other,  N  O,  acts  on  the  principle  of  the  single  microscope,  and 
is  called  the  eye-glass. 

There  is  another  kind  of  microscope,  called  the  solar  micro- 
scope, which  is  the  most  wonderful  from  its  great  magnifying 
power:  in  this  we  also  view  an  image  formed  by  a  lens,  not  the 
object  itself.  As  the  sun  shines,  I  can  show  you  the  effect  of 
this  microscope;  but  for  this  purpose,  we  must  close  the  shutters, 
and  admit  only  a  small  portion  of  light,  through  the  hole  in  the 
window -shutter,  which  we  used  for  the  camera  obscura.  We 
shall  now  place  the  object  A  B,  (plate  23,  fig.  1.)  which  is  a 
small  insect,  before  tlie  lens  C  D,  and  nearly  at  its  focus:  the 
image  E  F,  will  then  be  represented  on  the  opposite  wall,  in 
the  same  manner,  as  the  landscape  was  in  the  camera  obscura; 

24.  How  may  objects  be  ma^ified  without  the  aid  of  a  lens  ?  25.  Why 
can  an  object,  very  near  to  the  eye,  be  distinctly  seen,  when  viewed  through 
a  small  hole  ?  26.  Describe  the  double  microscope,  ai  represented  in  fig,  6, 
plate  22. 


202 


OPTICS. 


with  this  difference,  that  it  will  be  magnified,  instead  of  being 
diminished.  I  shall  leave  you  to  account  for  this,  bj  examinino- 
the  figure.  '^ 

Emily.  I  see  it  at  once.  The  image  E  F  is  magnified,  be- 
cause it  is  farther  from  the  lens,  than  the  object  A  B^  while  the 
representation  of  the  landscape  was  diminished,  because  it  was 
nearer  the  lens,  than  the  landscape  was.  A  lens,  then,  an- 
swers the  purpose  equally  well,  either  for  magnifying  or  dimin- 
ishiiig  objects? 

Mrs,  B.  Yes;  if  you  wish  to  magnify  the  image,  you  place 
the  object  near  the  focus  of  the  lens;  if  you  wish  to  produce  a 
diminished  image,  you  place  the  object  at  a  distance  from  the 
lens,  in  order  that  the  image  may  be  formed  in,  or  near  the 
focus. 

Caroline.  The  magnifying  power  of  this  microscope  is  pro- 
digious: but  the  indistinctness  of  the  image,  for  want  of  light,  is 
a  great  imperfection.  Would  it  not  be  clearer,  if  the  opening 
in  the  shutter  were  enlarged,  so  as  to  admit  more  light? 

Mrs.  B.  If  the  whole  of  the  light  admitted,  does  not  fall  upon 
the  object,  the  effect  will  only  be  to  make  the  room  lighter,  and 
the  image  consequently  less  distinct. 

Emily.  But  could  you  not  by  means  of  another  lens,  bring  a 
large  pencil  of  rays  to  a  focus  on  the  object,  and  thus  concen- 
trate upon  it  the  whole  of  the  light  admitted  ? 

Mrs.  B.  Very  well.  We  shall  enlarge  the  opening,  and 
place  the  lens  X  Y  (fig.  2.)  in  it,  to  converge  the  rays  to  a  focus 
on  the  object  A  B.  There  is  but  one  thing  more  wanting  to 
complete  the  solar  microscope,  which  I  shall  leave  to  Caroline's 
sagacity  to  discover. 

Caroline.  Our  microscope  has  a  small  mirror  attached  to  it, 
upon  a  moveable  joint,  which  can  be  so  adjusted  as  to  receive 
the  sun's  rays,  and  reflect  them  upon  the  object:  if  a  similar 
mirror  were  placed  to  reflect  light  upon  the  lens,  would  it  not 
be  a  means  of  illuminating  the  object  more  perfectly? 

Mrs.  B.     You  are  quite  ri^ht.     P  Q  (fig.  2.)  is  a  small  mir- 
ror, placed  on  the  outside  of  the  window-shutter,  which  receives 
the  incident  rays  S  S,  and  reflects  them  on  the  lens  X  Y.    Now 
that  we  have  completed  the  apparatus,  let  us  examine  the  mites 
on  this  piece  of  cheese,  which  I  place  near  the  focus  of  the  lens. 
Caroline.     Oh,  how  much  more  distinct  the  image  now  is,  and 
how  wonderfully  magnified!  The  mites  on  the  cheese  look  like  a 
drove  of  pigs  scrambling  over  rocks. 
Emily.     I  never  saw  any  thing  so  curious.     Now,  an  immense 
27.  How  does  the  solar  microscope,  (fig.  1  plate  23.)  operate  ?    28.  Why 
may  minute  objects  be  greatly  magnified  by  this  instrument  ?     29.  In  its  more 
perfect  form  it  has  other  appendages,  as  seen  in  fig.  2,  what  are  they  ?   and 
what  their  uses  ? 


OPTICS.  203 

piece  of  cheese  has  fallen :  one  might  imagine  it  an  earthquake : 
some  of  the  poor  mites  must  have  been  crushed;  how  fast  they 
run — the^  absolutely  seem  to  gallop. 

But  this  microscope  can  be  used  onlj  for  transparent  objects; 
as  the  light  must  pass  tlirough  them,  to  form  the  image  on  the 
wall? 

Mrs,  B.  Very  minute  objects,  such  as  are  viewed  in  a  mi- 
croscope, are  generally  transparent,  but  when  opaque  objects 
are  to  be  exhibited,  a  mirror  M  N  (fig.  3.)  is  used  to  reflect  the 
light  on  the  side  of  the  object  next  the  wall:  the  image  is  then 
formed  by  light  reflected  from  the  object,  instead  of  being 
transmitted  through  it. 

Emily.  Pray,  is  not  a  magic  lanthorn  constructed  on  the 
same  principles? 

Mrs.  B.  Yes,  with  this  difference;  the  objects  to  be  magni- 
fied, are  painted  upon  pieces  of  glass,  and  the  light  is  supplied 
by  a  lamp,  instead  of  the  sun. 

The  microscope  is  an  excellent  invention  to  enable  us  to  see 
and  distinguish  objects,  which  are  too  small  to  be  visible  to  tlie 
naked  eye.  But  there  are  objects,  which,  though  not  really 
small,  appear  so  to  us,  from  their  distance;  to  these,  we  cannot 
apply  the  same  remedy;  for  when  a  house  is  so  far  distant,  as  to 
be  seen  under  the  same  angle  as  a  mite  which  is  close  to.us,  the 
ettect  produced  on  the  retina  is  the  same :  the  angle  it  subtends 
is  not  large  enough  for  it  to  form  a  distinct  image  on  the  retina. 

Emily.  Since  it  is  impossible,  in  this  case,  to  make  the  object 
approach  the  eye,  cannot  we  by  means  of  a  lens  bring  an  im- 
age of  it,  nearer  to  us.^ 

Mrs.  B.  Yes;  but  then  the  object  being  very  distant  from  the 
focus  of  the  lens,  the  image  would  be  too  small  to  be  visible  to 
the  naked  eye. 

Emily.  Then,  why  not  look  at  the  image  through  another 
lens,  which  will  act  as  a  microscope,  enable  us  to  bring  the  im- 
age close  to  the  eye,  and  thus  render  it  visible? 

Mrs.  B.  Very  well,  Emily;  I  congratulate  you  on  having 
invented  a  telescope.  In  figure  4,  the  lens  C  D,  forms  an  image 
E  F,  of  the  object  A  B;  and  the  lens  X  Y,  serves  the  purpose  of 
magnifying  that  image;  and  this  is  all  that  is  required  in  a  com- 
mon refracting  telescope. 

Emily.  But  in  fig.  4,  the  image  is  not  inverted  on  the  retina, 
as  objects  usually  are:  it  should  therefore  appear  to  us  invert- 
ed; and  that  is  not  the  case  in  the  telescopes  I  have  looked 
through. 

30.  What  is  added  when  opaque  objects  are  to  be  viewed  ?  %.  3.  31.  In 
what  does  the  ma^c  lanthorn  dififer  from  the  solar  microscope  ^  32.  What 
are  the  use  and  structure  of  the  telescope,  as  shown  in  %.  4 ' 


204 


OPTICS. 


Mrs,  B.  When  it  is  necessary  to  represent  the  image  erect, 
two  other  lenses  are  required;  by  which  means  a  second  image  is 
formed,  the  reverse  of  the  first,  and  consequently  upright. 
These  additional  glasses  are  used  to  view  teiTestrial  objects,-  for 
no  inconvenience  arises  from  seeing  the  celestial  bodies  in- 
verted. 

Emily.  The  difference  between  a  microscope  and  a  telescope, 
seems  to  be  this> — a  microscope  produces  a  magnified  image,  be- 
cause the  object  is  nearest  the  lens;  and  a  telescope  produces  a 
diminished  image,  because  the  object  is  furthest  from  the  lens. 

Mrs.  B.  Your  observation  applies  only  to  the  lens  C  D,  or 
object-glass,  which  serves  to  bring  an  image  of  the  object  nearer 
the  eye;  for  the  lens  X  Y,  or  eye-glass,  is,  in  fact,  a  microscope, 
as  its  purpose  is  to  magnify  the  image. 

When  a  very  great  magnifying  power  is  required,  telescopes 
are  constructed  with  concave  mirrors,  instead  of  lenses.  These 
are  called  reflecting  telescopes,  because  the  image  is  reflected 
by  metallic  mirrors.  Concave  mirrors,  you  know,  produce  by 
reflection,  an  effect  similar  to  that  of  convex  lenses,  by  refraction. 
In  reflecting  telescopes,  therefore,  mirrors  are  used  in  order  to 
bring  the  image  nearer  the  eye;  and  a  lens,  or  eye-glass,  the  same 
as  in  the  refracting  telescope,  to  magnify  the  image. 

The  advantage  of  the  reflecting  telescope  is,  that  mirrors 
whose  focus  is  six  feet,  will  magnify  as  much  as  lenses  of  a  hun- 
dred feet:  dn  instrument  of  this  kmd  may,  therefore,  possess  a 
high  magnifying  power,  and  yet  be  so  short,  as  to  be  readily 
managed. 

Caroline.  But  I  thought  it  was  the  eye-glass  only  which 
magnified  the  image;  and  that  the  other  lens,  served  to  bring  a 
diminished  image  nearer  to  the  eye. 

Mrs.  B.  The  image  is  diminished  in  comparison  with  the 
object,  it  is  true;  but  it  is  magnified,  if  you  compare  it  to  the  di- 
mensions of  which  it  would  appear  without  the  intervention  of 
any  optical  instrument;  and  this  magnifying  power  is  greater  in 
reflecting,  than  in  refracting  telescopes. 

We  must  now  bring  our  observations  to  a  conclusion,  for  I 
have  communicated  to  you  the  whole  of  my  very  limited  stock 
of  knowledge  of  Natural  Philosophy.  If  it  enable  you  to  make 
further  progress  in  that  science,  my  wishes  will  be  satisfied;  but 
remember,  in  order  that  the  study  of  nature  may  be  productive 
of  happiness,  it  must  lead  to  an  entire  confidence  in  the  wisdom 
and  goodness  of  its  bounteous  Author. 

33.  When  terrestrial  objects  are  to  be  viewed,  why  are  two  additional 
lenses  employed  ?     34.  What  part  of  the  telescope  performs  the  part  of  a  mi- 
•roscope?     3.u.  la  what  does  the  reflecting,  differ  from  the  refractin*  tele- 
scope ?     36.  What  auvaatages,  do  reflectipg,  possess  over  refracting  t-^^^ 
scopes  ? 


GLOSSARY. 


\CCELERATED    MoTION.      Motion  IS 

said  to  be  accelerated,  when  the  ve- 
locity is  continually  increasing;. 

Accidental  Properties.  Those 
properties  of  bodies  which  are  lia- 
ble to  change,  as  colour,  form,  &c. 

Acute.: — See  Angle. 

Air.  Ar>  elastic  fluid.  The  atmo- 
sphere which  surrounds  the  earth, 
is  generally  understood  by  this 
term,  but  there  are  many  kinds  of 
air.  The  term  is  synonymous  with 
Gas. 

Air  PirMP.  An  instrument  by 
which  vessels  may  be  exhausted  of 
air. 

Altitude.  The  height  in  degrees  of 
the  sun,  or  any  heavenly  body, 
above  the  horizon. 

Angle.  The  space  contained  be- 
tween two  lines  inclined  to  each 
other,  and  which  meet  in  a  point. 
Angles  are  measured  in  degrees, 
upon  a  segment  of  a  circle  described 
by  fjlacing  ope  leg  of  a  pair  of  com- 
passes on  the  angular  point,  ai^d 
with  the  other,  describing  the  seg- 
ment between  the  two  lines.  If  the 
segment  be  exactly  l-4th  of  a  circle, 
it  is  called  a  right  angle,  and  con- 
tains 90  deg.  If  more  than  l-4th  of 
a  circle,  it  is  an  obtuse  angle.  If  less, 
an  acute  angle.     See  plate  2. 

Angle  of  Incidence,  is  the  space 
contained  between  a  ray  which  falls 
obliquely  upon  a  body,  and  a  line 
perpendicular  to  the  surface  of  the 
body,  at  the  point  where  the  ray  falls. 

Angle  op  Reflection.  The  space 
contained  between  a  reflected  ray, 
and  a  line  perpendicular  to  the  re- 
flecting point. 

Angle  op  Vision,  or  visual  angle. 
The  space  contained  between  lines 
drawn  from  the  extreme  parts  of 
any  object,  and  meeting  in  the  eye. 

AwTARCTic  Circle.  A  circle  ex- 
tending round  the  south  pole,  at  the 


distance  of  23  1-2  degrees  from  it. 
The  same  as  the  south  frigid  zone. 

Aphelion.  That  part  of  the  orbit  of 
a  planet,  in  which  its  distance  from 
the  sun  is  the  greatest. 

Area.  The  surface  enclosed  be- 
tween the  lines  which  form  the 
boundary  of  any  figure,  whether 
regular  or  irregular. 

Aries.     See  Sign. 

Asteroids.  The  name  given  to  the 
four  small  planets,  Ceres,  Juno,"* 
Pallas,  and  Vesta.  -», 

Astronomy.  The  science  which 
treats  of  the  motion  and  other  phe* 
nomena  of  the  sun,  the  planets,  the 
stars,  and  the  other  heavenly  bo- 
dies. 

Atmosphere.  The  air  which  sur- 
rounds the  earth,  extending  to  an 
unknown  height.  Wind  is  this  air 
in  motion. 

Attraction.  A  tendency  in  bodies 
to  approach  each  other,  and  to  exist 
in  contact. 

Attraction  op  Cohesion.  That 
attraction  which  causes  matter  to 
remain  in  masses,  preventing  them 
from  falling  into  powder.  For  this 
attraction  to  exist,  the  particles  must 
be  contiguous. 

Attraction  of  Gravitation.  By 
this  attraction,  masses  of  matter, 
placed  at  a  distance,  have  a  ten- 
dency to  approach  each  other.  At- 
traction is  mutual  between  the  sun 
and  the  planets. 

Axis  of  the  Earth,  or  of  any  oe 
the  planets.  An  imaginary  line 
passing  through  their  centres,  and 
terminating  at  their  poles  ;  round 
this  their  diurnal  revolutions  are 
performed. 

Axis  pp  Motion.  The  imaginary 
line,  around  which  all  the  parts  of  a 
body  revolve,  when  it  has  a  spin- 
ning motion. 

Axis  OB  A  L^NS,  OR  Mirror.     A 


i06 


GLOSSARY. 


line  passing  through  the  centre  of  a 
lens,  or  mirror,  in  a  direction  per- 
pendicular to  its  surface. 

Balloon.  Any  hollow  globe.  The 
term  is  generally  applied  to  those 
which  are  made  to  ascend  in  the  air. 

Barometer.  Commonly  called  a 
weather-glass.  It  has  a  glass  tube, 
containing  quicksilver,  which  by 
rfsing  and  falling,  indicates  any 
change  in  the  pressure  of  the  at- 
mosphere, and  thus  frequently 
warns  us  of  changes  in  the  wea- 
ther. 

Body.  The  same  as  Matter.  It 
may  exist  in  the  solid,  liquid,  or 
aeriform  state  ;  and  includes  every 
thing  with  which  we  become  ac- 
quainted by  the  aid  of  the  senses. 

Burning-glass,  or  Mirror.  A 
lens,  or  a  mirror,  by  which  the  rays 
of  light,  and  heat,  are  brought  to  a 
focus,  so  as  to  set  bodies  on  fire. 

Cambra  Obscura,  a  darkened  room; 
or  more  frequently  a  box,  admitting 
light  by  one  opening,  where  a  lens 
is  placed;  which,  bringing  the  rays 
of  light,  from  external  objects,  to  a 
focus,  presents  a  perfect  picture  of 
them,  in  miniature. 

Capillary  Tubes.  Tubes,  the 
bore  of  which  is  very  small.  Glass 
tubes  are  usually  employed,  to 
show  the  phenomenon  of  capillary 
attraction.  Fluids  in  which  tliey 
are  immersed,  rise  in  such  tubes 
above  the  level  of  that  in  the  con- 
taining vessel. 

Centre  of  a  Circle.  A  point, 
equally  distant  from  every  part  of 
its  circumference. 

Centre  of  Gravity.  That  point 
within  a  body,  to  which  all  its  par- 
ticles tend,  and  around  which  they 
exactly  balance  each  other.  A  sys- 
tem of  bodies,  as  the  planets,  may 
hav3  a  common  centre  of  gravity, 
around  which  they  revolve  in  their 
orbits ;  whilst  each,  like  the  earth, 
has  it3  particular  centre  of  gravity 
within  itself. 

Centre  of  Motion*  That  point 
about  which  the  parts  of  a  revolv- 
ing body  move,  which  point  is,  itself, 
considered  as  in  a  state  of  rest. 

Centre     of    MAttNiTUDE.      The 


middle  point  of  any  body.  Suppose 
a  globe,  one  side  of  which  is  formed 

"  of  lead,  and  the  other  of  wood,  the 
centres  of  magnitude  and  of  gravity, 
would  not  be  in  the  same  points. 

Central  Forces.  Those  which 
either  impel  a  body  towards,  or 
from,  a  centre  of  motion. 

Centrifugal.  That  which  gives  a 
tendency  to  fly  from  a  centre. 

Centripetal.  That  which  impels 
a  body,  towards  a  centre. 

Circle.  A  figure,  the  periphery,  or 
circumference  of  which,  is  every 
where  equally  distant,,  from  the 
point,  called  its  centre. 

Circle,  Great.  On  the  globe,  or 
earth,  is  one  that  divides  it  into  two 
equal  parts,  or  hemispheres.  The 
equator,  and  meridian  lines,  are 
great  circles. 

Circle  Lesser.  Those  which  di- 
vide the  globe  into  unequal  parts. 
The  tropical,  arctic  and  antarctic 
circles,  and  all  parallels  of  latitude, 
are  lesser  circles. 

Circumference.  The  boundary 
line  of  any  surface,  as  that  which 
surrounds  the  centre  of  a  circle  ;  the 
four  sides  of  a  square,  &c^ 

Comets.  Bodies  which  revolve 
round  the  sun,  in  very  long  ovals, 
approaching  him  very  nearly  in 
their  perihelion,  but  in  their  aphe- 
lion, passing  to  a  distance  immea- 
surably great. 

Cohesion.     See  Attraction. 

Compressible.  Capable  of  being 
forced  into  a  smaller  space. 

Concave.  Hollowed  out ;  the  inner 
surface  of  a  watch-glass  is  concave, 
and  may  represent  the  form  of  a 
concave  mirror^  or  lens. 

Convex,  Projecting,  or  bulging  out, 
as  the  exterior  surface  of  a  watch- 
glass,  which  may  represent  the 
form  of  a  convex  mirror,  or  lens. 

Cone.  A  body  somewhat  resembling 
a  sugar-loaf;  that  is,  having  a  round 
base,  and  sloping  at  the  sides,  until 
it  terminates  in  a  point. 

Conjunction.  When  three  of  the 
heavenly  bodies  are  in  a  straight  or 
right  line,  if  you  take  either  of  the 
extreme  bodies,  the  otlier  two  are 
ia  conjunction  with  it;  because  a 


GLOSSARY. 


207 


straight  line  drawn  fr6m  it,  might 
pass  through  the  centres  of  both, 
and  join  them  together.  At  the 
time  of  new  moon,  the  moon  and 
sun  are  in  conjunction  with  the 
earth  ;  and  the  moon  and  earth,  are 
in  conjunction  with  the  sun. 

CoNSTELLA-TidN,  OR  SiGN.  A  col- 
lection of  stars.  Astronomers  have 
imagined  pictures  drawn  in  the  hea- 
vens, so  as  to  embrace  a  number  of 
contiguous  stars,  and  have  named 
the  group  after  the  animal,  or  other 
article  supposed  to  be  drawn;  an 
individual  star  is  generally  desig- 
nated by  its  fancied  location ;  as 
upon  the  ear  of  Z.eo,  the  Lion,  &c. 

Convergent  Rays,  are  those 
which  approach  each  other,  so  as 
eventually  to  meet  in  the  same 
point. 

Crystals.  Bodies  of  a  regular  form, 
having  flat  surfaces,  and  well  defin- 
ed angles.  Nitre,  and  other  salts, 
are  familiar  examples.  Many  mass- 
es of  matter,  are  composed  of  crys- 
tals too  minute  to  be  discerned  with- 
out glasses. 

Curvilinear,  consisting  of  a  line 
which  is  not  straight,  as  a  portion  of 
a  circle,  of  an  oval,  or  any  curved 
line. 

Cylinder.  A  body  in  the  form  of  a 
roller,  having  flat  circular  ends,  and 
being  of  equal  diameter  throughout 

Degree.  If  a  circle  of  any  size  be 
divided  into  360  equal  parts,  each 
of  these  parts  is  called  a  degi-ee. 
One  quarter  of  a  circle  contains 
ninety  degrees ;  one  twelfth  of  a 
circle,  thirty  degrees.  The  actual 
length  of  a  degree,  must  depend 
upon  the  size  of  the  circle.  A  de- 
gree upon  the  equator,  uix)n  a  me- 
ridian, or  any  great  circle  of  the 
earth,  is  equal  to  69  1-2  miles. 

Straight  lines  are  sometimes  divided 
into  equal  parts,  called  degrees;  but 
these  divisions  are  arbitrary,  bear- 
ing no  relationship  to  the  degrees 
upon  a  circle. 

Density.  Closeness  of  texture.  When 
two  bodies  are  equal  in  bulk,  that 
wliich  weighs  the  most,  has  the 
greatest  density. 

Diagonal.   A  line  drawn  so  as  to 


connect  two  remote  angles    of  a 
square,  or  other  four-sided  figure. 

Dilatation.  The  act  of  increasing 
in  size.  Bodies  in  general,  dilate 
when  heated,  and  contract  by  cool- 
ing. 

Discord.  When  the  vibrations  of 
the  air,  produced  by  two  musical 
tones,  do  not  bear  a  certain  ratio  to 
each  other,  a  jarring  sound  is  pro- 
duced, which  is  called  discord. 

Divergent  Rays.  Those  which 
proceed  from  tlie  same  point,  but  are 
continuallyreceding  from  each  other. 

Divisibility.  Capability  of  being 
divided,  or  of  having  the  parts  sepa- 
rated from  each  other.  This  is 
called  one  of  the  essential  properlics 
of  matter;  because,  however  minute 
the  particles  may  be,  they  must  still 
contain  as  many  halves,  quarters, 
&;c.  as  the  largest  mass  of  matter. 

Echo.  A  sound  reflected  back,  by 
some  substance,  so  situated  as  to 
produce  this  eifect. 

Eclipse.  The  interruption  of  the 
light  of  the  sun,  or  of  some  other 
heavenly  body,  by  the  iiUervention 
of  an  opaque  body.  The  moon 
passing  between  the  earth  and  tlie 
sun,  causes  an  eclipse  of  the  latter. 

Ecliptic.  A  circle  in  the  heavejis. 
The  apparent  path  of  the  sun, 
through  the  twelve  signs  of  the  zo- 
diac. This  is  caused  by  the  actual 
revolution  of  the  earth,  round  the 
sun.  It  is  called  the  ecliptic,  be- 
cause eclipses  always  happen  in  the 
direction  of  that  line,  from  the  earth. 

Elasticity.  That  property  of  bo- 
dies, hy  which  they  resume  their 
dimensions  and  form,  when  the  force 
which  changed  them  is  removed. 
Air  is  eminently  elastic.  Two  ivory 
balls,  struck  together,  become  flat- 
tened at  the  point  of  contact ;  but 
immediately  resuming  their  form, 
they  react  upon  each  other. 

Ellipsis.  An  oval.  This  figure  dif- 
fers from  a  circle,  in  being  uneqaal 
in  its  diameters,  and  in  having 
two  centres,  or  points,  called  its/oa. 
The  orbits  of  the  planets  are  all  el- 
liptical. 

EauATOR.  That  imaginary  lino 
which  divides  the  earth  into  north- 


208 


GLOSSARY. 


em  and  southern  hemispheres,  and 
.which  is  equally  distant  from  each 
pole. 
EauiLiBRiUM.  When  two  articles 
exactly  balance  each  other,  they  are 
in  equilibrium.  They  may,  not- 
withstanding-, be  very  unequal  in 
weight,  but  they  must  be  so  situat- 
ed, that,  if  set  in  motion,  their  mo- 
mentums  would  be  equal. 

EauiNox.  The  two  periods  of  time  at 
which  the  nights  and  days  are  every 
where  of  equal  length.  Ttae  vernal 
equinox  is  in  March,  when  the  sun 
enters  the  sign  Aries;  the  autumnal 
equinox  in  September,  when  the  sun 
enters  Libra.  At  these  periods,  the 
sun  is  vertical  at  the  equator. 

Exhalations.  All  those  articles 
which  arise  from  the  earth,  and 
mixing  with  the  atmosphere,  form 
vapour. 

Expansion.  The  same  as  dilatation, 
which  see. 

Extension.  One  of  the  essential 
properties  of  matter ;  that  by  which 
it  occupies  some  space,  to  the  exclu- 
sion of  all  other  matter. 

''"■iGURE.  All  matter  must  exist  in 
some  form,  or  shape;  hence  figure  is 
defemed  an  essential  proji^rty  of 
matter. 

■p'LUiD.  A  form  of  matter,  in  which 
its  particles  readily  flow,  or  slide, 
over  each  other.  Airs,  or  gases, 
are  called  elastic  fluids,  because 
they  are  readily  reduced  to  a  small- 
er bulk  by  pressure.  Liquids,  are 
denominated  non-elastic  fluids,  be- 
cause they  suffer  but  little  diminu- 
tion of  bulk,  by  any  mechanical 
force. 

Focus.  That  point  in  which  con- 
verging rays  unite. 

Force.  That  power  which  acts 
upon  a  body,  either  tending  to  cre- 
ate, or  to  stop  motion. 

FcxxNTAiN.  A  jet,  qr  stream  of  wa- 
ter, forced  upwards  by  the  weight  of 
other  water,  by  the  elasticity  of  air, 
or  some  other  mechanical  pressure. 

Friction.  The  rubbing  of  bodies 
together,  by  which  their  motion  is 
retarded.  Friction  maybe  lessened, 
but  cannot  be  destroyed. 

Frigid    Zones.      The    spaces     or 


areas,  contained  within  the  arctic 
and  antarctic  circles. 

Fulcrum.  A  prop.  The  point  or  axis, 
by  which  a  body  is  supported,  and 
about  which  it  is  susceptible  of  mo- 
tion. 

Gas.  Any  kind  of  air  ;  of  these  there 
are  several.  The  atmosphere  con- 
sists of  two  kinds,  mixed,  or  com- 
bined with  each  other. 

Geometry.  That  branck  of  tlie 
mathematics,  which  treats  of  lines, 
of  surfaces,  and  of  solids;  and  inves- 
tigates their  properties,  and  pro- 
portions. 

Globe.  A  sphere,  or  ball.  It  has 
a  point  in  its  centre  of  magnitude, 
from  which  its  surface  is  every 
where  equally  distant. 

Gravity.  That  species  of  attrac- 
tion which  appears  to  be  common 
to  matter,  existing  in  its  particles, 
and  giving  to  them,  and  of  course 
to  the  masses  which  thoy  compose, 
a  tendency  to  approach  each  other. 
By  gravity  a  stone  falls  to  the  earth, 
and  by  it  the  heavenly  bodies  tend 
towards  each  other. 

Harmony.  A  combination  of  musi- 
sical  sounds,  produced  by  vibra- 
tions which  bear  a  certain  ratio  to 
each  other ;  and  which  thence  ai- 
fect  the  mind  agreeably,when  heard 
at  the  same  time.  Sounds  not  so 
related,  produce  discord. 

Hemisphere.  Half  a  sphere  or 
globe.  A  plane  passing  through  the 
centre  of  a  globe,  will  divide  it  into 
hemispheres. 

Horizon,  'l^his  is  generally  divided 
into  sensible^  and  rational.  The 
sensible  horizon  is  that  portion  of 
the  surface  of  the  earth,  to  which 
our  vision  extends.  Our  rational 
horizon  is  that  circle  in  the  heavens 
which  bounds  our  vision,  when  on 
the  ocean,  an  extended  plane,  or  any 
elevated  situation.  In  the  heavens 
our  sensible,  and  our  rational  hori- 
zon are  the  same ;  its  plane  would 
divide  the  earth  into  hemispheres 
at  90  degrees  from  us  ;  and  a  per- 
son standing  on  that  part  of  the 
earth  which  is  directly  opposite  to 
us,  would,  at  the  same  moment,  see 
in  his  horizon,  the  same  heavenly 


«LOSSARY. 


209 


bodies,  which  would  be  seen  in 
ours. 

HoRizoif  TAL.  Level;  not  inclined,  or 
sloping.  A  perfectly  round  ball, 
placed  upon  a  flat  surface,  which 
is  placed  horizontally,  will  remain 
at  rest. 

Hydraulics.  That  science  which 
treats  of  water  in  motion,  and  the 
means  of  raifiing,  conducting,  and 
using  it  for  moving  machinery,  or 
other  purposes. 

Hydrostatics.  Treats  of  the 
weight,  pressure,  and  equilibrium 
of  fluids,  when  in  a  state  of  rest. 

Hydrometer.  An  instrument  used 
to  ascertain  the  specific  gravity  of 
difierent  fluids,  which  it  does,  by 
the  depth  to  which  it  sinks  when 
floating  on  them. 

Image.  The  picture  of  any  object 
which  we  perceive  either  by  re- 
flected or  refracted  light.  All  ob- 
jects which  ai-e  visible,  become  so 
by  forming  images  on  the  re- 
tina. 

Impenetrability,  l^hat  property 
of  matter,  by  which  it  excludes  all 
other  matter  from  occupying  the 
same  space  with  itself  at  the  same 
time.  If  two  particles  could  exist 
in  the  same  space,  so  also  might  any 
greater  number,  and  indeed  all  the 
matter  in  the  universe,  might  be 
collected  in  a  single  point. 

Incidence.  The  direction  in  which 
a  body,  or  a  ray  of  light,  pioves  in 
its  approach  towards  any. substance, 
upon  which  it  strikes. 

JifCLiJVED  Plane.  One  of  the  six 
mechanical  powers.  Any  platie 
surface  inclined  to  the  horizon,  may 
be  so  denominated. 

Inertia.  One  of  the  inherent  pro- 
perties of  matter.  Want  of  power, 
or  of  any  active  principle  witliin  it- 
self, by  which  it  can  change  its  own 
state,  whether  of  motion,  or  of  rest. 

Ikherent  Properties.  Those 
properties  which  are  absolutely  ne- 
cessary to  the  esdstence  of  a  body  ; 
called  also  essential  properties.  All 
others  are  denominated  accidental. 
Colour  is  an  accidental — extension, 
an  esseutial  property  of  matter. 

Latitude.  Distance  from  the  equa- 
S2 


tor,  in  a  direct  line  towards  either 
pole.  This  distance  is  measured  in 
degrees  and  minutes.  The  degree 
of  latitude  cannot  exceed  ninety,  or 
one  quarter  of  a  circle.  Places  to 
the  south  of  the  equator,  are  in 
south  latitude,  and  those  to  the 
north,  in  north  latitude. 

Latitude,  Parallels  of.  Lines 
drawn  upon  the  globe,  parallel  to 
the  equator,  are  so  called  ;  every 
place  situated  on  such  a  line,  has 
the  same  latitude,  because  equally 
distant  from  the  equator. 

Lens.  A  glass,  ground  so  that  one  or 
both  surfaces  form  segments  of  a 
sphere,  serving  either  to  magnify,  or 
diminish  objects  seen  through  them. 
Glasses  used  in  spectacles  are  lenses'. 

Lever.  One  of  the  mechanical  pow- 
ers. An  inilexible  bar  of  wood  or 
metal,  supported  by  a  fulcrum,  or 
prop ;  and  employed  to  increase 
the  efiect  of  a  given  power. 

Libra.  One  of  the  twelve  signs  of 
the  zodiac.  That  into  which  the  sun 
enters,  at  the  autumnal  equinox. 

LiGRT.  That  principle,  by  the  aid 
of  which  we  are  able  to  discern  all 
visible  objects.  It  is  generally  be- 
lieved to  be  a  substance  emitted  by 
luminous  bodies,  and  exciting  vision 
by  passing  into  the  eye. 

Longitude.  Distance  measured  in 
degrees  and  minutes,  either  in  an 
eastern,  or  a  western  direction,  from 
any  given  point  either  on  the  equa- 
tor, or  on  a  parallel  of  latitude. 
Degrees  of  longitude  may  amount 
to  180,  or  half  a  circle.  A  degree 
of  longitude  measured  upon  the 
equator,  is  of  the  same  length  with 
a  degree  of  latitude;  but  as  the 
poles  are  approached,  the  degrees 
of  longitude  diminish  in  length,  be- 
cause the  circles  upon  which  they 
are  measured,  become  less. 

Lunar.  Relating  to  Luna,  the 
moon. 

Lunation.  The  time  in  which  the 
moon  completes  its  drcuit.  A  lunar 
month. 

Luminous  Bodies.  Those  which 
emit  light  from  their  own  substance; 
not  shining  by  borrowed,  or  reflect- 


210 


GLOSSARY. 


Ma^chine.  Any  instrument,  either 
simple  or  compound,  by  which  any 
mechanical  effect  is  produced.  A 
needle,  and  a  clock,  are  both  ma- 
chines. 

Ma»ic  Lanthorn,  or  Lantern. 
An  optical  instrument,  by  which 
transparent  pictures,  painted  upon 
glass,  are  magnified  and  exhibited 
on  a  white  wall  or  screen,  in  a  dark- 
ened room.  The  phantasmagoria, 
is  a  species  of  magic  lanthorn. 

Mathematics.  The  science  of 
numbers  and  of  extension.  Com- 
mon arithmetic,  is  a  lower  branch 
of  the  mathematics.  In  its  higher 
departments,  it  extends  to  every 
thing  which  is  capable  of  being  ei- 
ther numbered  or  measured. 

Matter.  Substance.  Every  thing 
with  which  we  become  acquainted 
by  the  aid  of  the  senses ;  every  thing 
however  large,  or  however  minute, 
which  has  length,  breadth,  and 
thickness. 

Mechanics.  That  science  which 
investigates  the  principles,  upon 
which  the  action  of  every  machine 
depends  ;  and  teaches  their  proper 
application  in  overcoming  resist- 
ance, and  in  producing  motion,  in  all 
the  useful  purposes  to  which  they 
are  applied. 

Medium.  In  optics,  is  any  body 
which  transmits  light.  Air,  water, 
glass,  and  all  other  transparent  bo- 
dies, are  media.  Medium  also  de- 
notes that  in  which  any  body  moves. 
Air  is  the  medium  which  conveys 
sound,  and  which  enables  birds  to  fly. 

Melody.  A  succession  of  such  single 
musical  sounds,  as  form  a  simple  air 
or  tune. 

Mercury.  That  planet  which  is 
nearest  to  the  sun.  Quicksilver,  a 
metal,  which  remains  fluid  at  the 
common  temperature  of  the  atmo- 
sphere. It  is  capable  of  being  ren- 
dered! solid,  by  intense  cold. 

Meridian.  Midday.  A  meridian  line, 
is  one  which  extends  directly  from 
one  pole  of  the  earth  to  the  other  ; 
crossing  the  equator  at  right  angles. 
It  is  therefore  half  of  a  great  circle. 
The  hour  of  the  day  is  the  same  at 


every  place  situated  on  the  same  me- 
ridian. Longitude  is  measured  from 
any  given  meridian,  to  the  oppo- 
site meridian.  Places  at  the  same 
distance  in  degrees,  to  the  east  or 
west  of  any  meridian,  have  the 
same  longitude. 

Microscope.  An  optical  instrument, 
by  which  mirmte  objects,  are  mag- 
nified, so  as  to  enable  us  to  perceive 
and  examine  such  as  could  not  be 
seen  by  the  naked  eye. 

Mineral.  Earths,  stones,  metals, 
salts,  and  in  general  all  substances 
dug  out  of  the  earth,  are  denomi- 
nated minerals. 

Minute.  In  time,  the  sixtieth  part 
of  an  hour.  In  length,  the  sixtieth 
part  of  a  degree.  A  minute  of  time, 
is  an  unvarying  period;  bat  a  minute 
in  length  varies  in  extent,  with  tiie 
degree  of  which  it  forms  a  part. 
The  degrees  and  minutee  are  equal 
in  number,  upon  a  common  ring, 
upon  the  equator  of  the  earth,  or, 
on  any  circle  of  the  heavens. 

Mirrors.  Polished  surfaces  of  metal, 
or  of  glass  coated  with  metal,  for 
the  purpose  of  reflecting  the  rays  of 
light,  and  the  images  of  objects. 
Common  looking-glasses,  are  mir- 
rors. Those  used  in  reflecting  te- 
lescopes, are  made  of  metal. 

Mobility.  Capable  of  being  moved 
from  one  place  to  another.  This  is 
accounted  one  of  the  essential  pro- 
perties of  matter,  because  we  can- 
not conceive  of  its  existence  without 
this  capacity. 

Momentum.  The  force,  or  power, 
with  which  e.  body  in  motion  acts 
upon  any  other  body,  or  tends  to 
preserve  its  own  quantity  of  mo- 
tion. The  momentum  of  a  body,  is 
compounded  of  its  quantity  of  mat- 
ter, and  its  velocity.  A  body  weigh- 
ing one  pound,  moving  with  a  velo- 
city of  two  miles  in  a  minute,  will 
possess  the  same  momentum  with 
one  weighing  two  pounds,  moving 
with  a  velocity  of  one  mile  in  a  mi- 
nute. 

Motion.  A  continued  and  success- 
ive change  of  place,  either  of  a 
whole  body,  or  of  the  particles  of 


GLOSSARY. 


211 


which  a  body  is  composed ;  the 
earth  in  revolving  upon  its  axis 
only,  would  not  change  its  place  as  a 
body,  but  all  the  particles  of  which 
it  is  co;npo3ed,  would  revolve 
round  a  common  axis  of  motion.  In 
revolving  in  its  orbit,  its  whole 
mass  is  constantly  occupying  a  new 
portion  of  space. 

Natural  Philosophy.  That  sci- 
ence which  enquires  into  the  laws 
which  govern  all  the  natural  bodies 
in  the  universe,  in  all  their  changes 
of  place,  or  of  state. 

Neap  Tides,  Those  tides  which 
occur  when  the  moon  is  in  her 
quadiatures,  or  half  way  between 
new,  and  full  moon;  at  these  pe- 
riods the  tides  are  the  lowest. 

Nodes.  Those  points  in  the  orbit  of 
the  moon,  or  of  a  planet,  where  it 
crosses  the  ecliptic  or  plane  of  the 
earth's  orbit.  When  passing  to  the 
north  of  the  ecliptic,  it  is  called  the 
ascending  node ;  wh^n  to  tlie  south 
of  it,  the  descending  node. 

Oblate.     See  Spheroid. 

OcTAGOKT.  A  figure  with  eight  sides, 
and  consequently  with  eight  angles. 

OPAauE.  Not  transparent ;  refusing 
a  passage  to  the  rays  of  light. 

Optics.  That  branch  of  science 
which  treats  of  light,  and  vision.  It 
is  generally  divided  into  two  parts. 
Catoptrics,  which  treats  of  the  re- 
flection of  light,  and  Dioptrics, 
which  treats  of  its  refraction. 

Orbit.  The  line  in  which  a  primary 
planet  moves  in  its  revolution  round 
the  sun ;  or  a  secondary  planet,  in 
its  revolution  round  its  primary. 
These  orbits  are  all  elliptical,  or  oval. 

Parabola.  A  particular  kind  of 
curve  ;  that  which  a  body  describes 
in  rising  and  in  falling, when  thrown 
upwards,  in  any  direction  not  per- 
pendicular teethe  horizon. 

Parallelogram,  A  figure  with 
four  sides,  having  those  which  are 
opposite,  parallel  to  each  other.  A 
square,  an  oblong  square,  and  the 
figure  usually  called  a  diamond,  are 
Parallelograms. 

Parallel  Lines.  All  lines,  whe- 
ther straight  or  curved,  which  are 


every  where  at  an  equal  distance 
from  each  other,  are  parallel  lines. 

Parallel  oe  Latitude.  See  La- 
titude. 

Perihelion.  That  part  of  the  orbit 
of  a  planet,  in  which  it  approaches 
the  sun  most  nearly. 

Pendulum.  A  body  suspended  by 
a  rod,  or  line,  so  that  it  may  vi- 
brate, or  oscillate,  backwards  and 
forwards.  Pendulums  of  the  same 
length,  perform  their  vibrations  in 
the  same  time,  whatever  may  be 
their  weight,  and  whether  Uie  arc 
of  vibration,  be  long  or  short. 

Percussion.     The  striking  of  bodies 
against  each, other.     The  force  of. 
this,  depends  upon  the  momentum 
of  the  striking  body. 

Period.  The  time  required  for  the 
revolution  of  one  of  the  neavenly 
bodies  in  its  orbit. 

Perpendicular.  Making  an  angle 
of  90  degrees  with  the  horizon. 
When  two  lines  which  meet,  make 
an  angle  of  90  degrees,  they  are 
perpendidicular  to  each  other. 

Phases.  The  various  appearances 
of  the  disc,  or  face  of  the  moon,  and 
of  the  planets;  that  portion  of 
them  which  we  see  illuminated  by 
the  rays  of  the  sun. 

Phenomenon.  Any  natural  appear- 
ance is  properly  so  called;  the  term, 
however,  is  usually  applied  to  ex- 
traordinary appearances,  as  eclip- 
ses, transits,  &c. 

Piston.  That  part  of  a  pump,  or 
other  engine  which  is  made  to  fit 
into  a  hollow  cylinder,  or  barrel ; 
and  to  move  up  and  down  in  it,^ 
in  order  to  raise  water,  or  for  any 
otlier  purpose. 

Plane.  A  perfectly  flat  surface. 
The  plane  of  the  orbit  of  a  planet, 
is  an  imaginary  flat  surface,  extend- 
ing to  every  part  of  the  orbit. 

Planet.  Those  bodies  which  re- 
volve round  the  sun,  in  orbits  near- 
ly circular.  They  are  divided  into 
primary,  and  secondary ;  these  lat- 
ter are  also  called  satellites,  or 
moons  ;  they  revolve  round  the  pri- 
mary planets,  and  accompany  them 
in  their  courses  round  the  sun. 


212 


GLOSSARY. 


Plitmb-hne.  a  string,  or  cord,  by 
which  a  weight  is  suspended  ;  it  is 
used  foi*  the  purpose  of  finding  a 
line  perpendicular  to  the  horizon ; 
the  weight  being  always  attracted 
towards  the  centre  of  the  earth. 

Pneumatics.  That  branch  of  natu- 
ral philosophy,  which  treats  of  the 
mechanical  properties  of  the  atmo- 
sphere, or  of  air  in  general. 

Poles.  The  extremities  of  the  axis  of 
motion  either  of  our  earth,  or  of  any 
other  revolving  sphere.  The  poles 
of  the  earth  have  never  been  visit- 
ed; the  regions  by  which  they  are 
surrounded,  being  obstructed  by 
impassable  barriers  ot  ice. 

Power.  That  force  which  we  apply 
to  any  mechanical  instrument,  to 
effect^  given  purnose,  is  denomi- 
nated power,  from  whatever  source 
it  may  be  derived.  We  have  the 
power  of  weights,  of  springs,  of  hor- 
ses, of  men,  of  steam,  &c. 

Prism.  The  instrument  usually  so 
called,  is  employed  in  optics  to  de- 
compose the  solar  ray:  it  consists  of 
a  piece  of  solid  glass,  several  inches 
in  length,  and  having  three  flat  sides; 
the  ends  are  equal  in  siie,  and  are 
of  course  triangular. 

Precession  of  the  EairmoxES. 
Every  equinox  takes  place  a  few 
seconds  of  a  degree,  before  the  earth 
arrives  at  that  part  of  the  ecliptic 
in  .which  the  preceding  equinox  oc- 
curred. This  phenomenon  is  called 
the  precession  of  the  equinoxes. 
There  is  consequently  a  gradual 
change  of  the  places  of  the  signs  of 
the  zodiac :  a  fact,  the  discovery  of 
which  has  thrown  much  light  on 
ancient  chronology. 

Projection.  That  force  by  which 
motion  is  given  to  a  body,  by  some 
power  acting  upon  it,  independ- 
ently of  gravity. 

Pulley.  One  of  the  six  mechanical 
powers.  A  wheel  turning  upon  an 
axis,  with  a  line  passing  over  it.  It 
is  the  moveable  pulley  only,  which 
gives  any  mechanical  advantage. 

Pump.  An  hydraulic,  or  pneumatic 
instrument,  for  the  purpose  of  rais- 
ing water,  or  exhausting  air. 

Quadrant.     A  quarter  of  a  circle. 


An  instrument  used  to  measure  the 
elevation  of  a  body  in  degrees  above 
the  horizon. 

Qdadratures  of  the  Moon. 
That  period  in  which  she  appears 
in  the  form  of  a  semicircle.  She  is 
then  either  in  her  first,  or  her  last 
quarter ;  and  exactly  half  way,  be- 
tween the  places  of  new,  and  of 
full  moon. 

Radiation.  The  passage  of  light  or 
heat  in  rays,  or  straight  lines ;  these 
being  projected  from  every  lumi- 
nous, or  heated  point,  in  all  direc- 
tions. 

Radius.  The  distance  from  the  cen- 
tre of  a  circle,  to  its  circumference; 
or  one  half  of  its  diameter.  In  the 
plural  denominated  radii. 

Rainbow.  An  appearance  in  the  at- 
mosphere, occasioned  by  the  de- 
composition of  solar  light,  in  its  re- 
fraction, and  reflection,  in  passing 
through  drops  of  i-ain.  The  bow 
can  be  seen,  only  when  the  sun  is 
near  the  horizon,  when  the  back 
is  turned  towards  it,  and  there  is  a 
shower  in  the  oppoite  direction. 

Ray.  a  single  line  of  light,  emitted 
in  one  direction,  from  any  luminous 
point. 

Reaction.  Every  body,  whether  in 
a  state  of  motion,  or  at  rest,  tende 
to  remain  in  such  state,  and  resists 
the  action  of  any  other  body  upon 
it,  with  a  force  equal  to  that  action. 
This  resistance,  is  called  its  re- 
action. 

Receiver.  This  name  is  applied 
to  glass  vessels  of  various  kinds,  ap- 
pertaining to  the  air  pump,  and 
from  which  the  air  maybe  exhaust- 
ed. They  are  made  to  contain,  or 
receive,  any  article  upon  which  an 
effect  is  to  be  produced,  by  taking 
off  the  pressure  of  the  atmosphere. 

Refraction,  of  the  rays  of  light,  is 
the  bending  of  those  rays,  when 
they  pass  obliquely  from  one  medi- 
um into  another  of  different  density. 
A  stick  held  obliquely  in  water,  ap- 
pears bent  or  broken  at  the  surface 
of  the  fluid. 

Refrangibility.  Capacity  of  be- 
ing refracted.  Light  ia  decomposed 
by  the  prism,  because  its  compo- 


GLOSSARY. 


213 


nent  parts  are  refrangible  in  differ- 
ent degrees,  by  the  same  refracting 
medium. 

REPtTLSiow.  The  reverse  of  attrac- 
tion. A  tendency  in  particles,  or  in 
masses  of  matter,  to  recede  from 
each  other.  The  matter  of  heat 
within  a  body,  appears  to  counter- 
act the  attraction  of  its  particles,  so 
as  to  prevent  absolute  contact. 

RETiJ»fA.  That  part  of  the  ball  of 
the  eye,  upon  which  the  images  of 
visible  objects  are  formed;  and  from 
which,  the  idea  of  such  forms,  is 

1     conveyed  to  the  mind. 

Revolution,  of  a  planet ;  is  either 
diurnal,  or  annual ;  the  former,  is 
its  turning  upon  its  own  axis;  the 
latter,  is  its  passage  in  its  orbit. 

Satellites.  Moons,  secondary 
planets. 

Segment  of  a  circle.  A  portion, 
or  part  of  a  circle  ;  called  also,  an 
arc  of  a  circle. 

Semi-diameter.  Half  the  diame- 
ter. The  semi-diameter  of  the 
earth,  is  the  distance  from  its  sur- 
face, to  its  centre. 

SiBERiAL.  Belonging  to  the  stars. 
A  siderial  day,  is  the  time  required 
for  a  star  to  reappear  on  a  given 
meridian.  A  siderial  year,  the 
period  in  which  the  sun  appears  to 
have  travelled  round  the  ecliptic, 
so  as  to  have  arrived  opposite  to 
any  particular  star,  from  which  his 
course  was  calculated. 
*  Signs,  or  Constellations.  Col- 
lections, or  groups,  of  stars.  Those 
of  the  zodiac  are  twelve,  corres- 
ponding with  the  twelve  months  in 
the  year.  In  the  centre  of  these 
the  ecliptic  is  situated.  The  sun 
appears  to  pass  in  succession  tlirough 
these  signs ;  entering  the  first  de- 
gree of  aries,  which  is  accounted  the 
first  sign,  about  the  21st  of  March. 

Sky.  That  vast  expanse,  or  space, 
in  which  the  heavenly  bodies  «are 
situated.  Its  blue  appearance  is 
supposed  to  arise  from  the  particles 
of  which  the  atmosphere  is  compos- 
ed, possessing  tlie  property  of  re- 
flecting the  blue  rays,  in  greatest 
abundance. 

Sola?,.     Appertaining  tp,  or  govern- 


ed by,  the  sun :  as  the  solar  sys- 
tem, the  solar  year,  solar  eclip- 
ses. 

Solid.  Not  fluid.  Having  its  parts 
connected  so  as  to  form  a  mass.  So- 
lid bodies,  are  not  absolutely  so,  all 
undoubtedly  containing  pores,  or 
spaces  void  of  matter. 

Solstices.  The  middle  of  summer 
and  the  middle  of  winter ;  those 
two  points  in  the  orbit  of  the  earth, 
in  which  its  poles  point  most  di-. 
rectly  towards  the  sun. 

Sonorous  Bodies.  Those  bodies 
which  are  capable  of  being  put  into 
a  state  of  vibration,  so  as  to  emit 
sounds. 

Specific  Gravity.  The  relative 
weight  of  bodies  of  different  spe- 
cies, when  the  same  bulk  of  each  ia 
taken.  Water  has  been  chosen  as 
the  standard  for  comparison.  If  we 
say  that  the  specific  gravity  of  a 
body  is  6,  we  mean,  that  its  weight 
is  six  times  as  great  as  that  of  a 
portion  of  water,  exactly  equal  to  it 
in  bulk. 

Spectrum.  That  appearance  of 
differently  coloured  rays,  which  is 
produced  by  the  refraction  of  the 
solar  ray,  by  means  of  a  prism,  ia 
called  the  prismatic  spectrum  ;  it 
exhibits  most  distinctly,  and  beauti- 
fully, all  the  colours  seen  in  the 
rainbow. 

Sphere.     A  globe,  or  ball. 

Spheroid.  Spherical ;  a  body  ap- 
proaching nearly  to  a  sphere  in  its 
figure.  The  earth,  is  denominated 
an  oblate  sphermd ;  it  not  being  an 
exact  sphere,  but  flattened  at  the 
poles,  so  as  to  cause  the  polar  di^ 
ameter  to  be  upwards  of  thirty 
miles  less  than  the  equatorial.  Ob- 
late, is  the  reverse  of  oblong,  and 
means  shorter  in  one  direction,  than 
in  another. 

Spring  Tides.  Those  tides  which 
occur  at  the  time  of  new,  or  of  full 
moon.  The  tides  then  rise  to  a 
greater  height  than  at  any  other 
period. 

SauARE.  A  figure  having  four  sides 
of  equal  length,  and  its  angles  all 
right  angles. 

In  numbers ;  the  product  of  a  number 


214 


GLOSSARY. 


multiplied  into  itself;  thns,  the 
square  of  3  is  9,  and  the  square  of  8 
is  64. 

Star.  The  fixed  stars  are  so  called, 
because  they  retain  their  relative 
situations;  while  the  planets,  by  re- 
volving in  their  orbits,  appear  to 
wander  amongst  the  fixed  stars. 

Subtend.  This  term  is  applied  to 
the  measurement  of  an  angle;  when 
the  lines  by  which  it  is  bounded 
recede  but  little  from  each  other, 
they  are  said  to  subtend  ;  that  is, 
to  be  contained  under,  a  small  an- 
gle. 

SirPERFiciES.  The  surface  of  any 
figure.  Space  extended  in  length 
and  width. 

System.  The  mutual  connexion, 
and  dependance  of  things,  upon 
each  other.  The  solar,  or  Coper- 
nican  system,  includes  the  sun,  the 
planets,  witli  their  moons,  and  the 
comets. 

Tawgewt.  a  straight  line  touching 
the  circumference  of  a  circle ;  but 
which  would  not  cut  off  any  portion 
of  it,  were  it  extended  beyond  the 
touching  point,  in  botli  directions. 

Telescope.  An  instrument  by 
which  distant  objects  may  be  dis- 
tinctly seen ;  the  images  of  ob- 
jects being  brought  near  to  the  eye, 
and  greaUy  magnified. 

Temperate  Zones.  Those  portions 
of  the  surface  of  the  earth  situated 
between  23  1-2  and  66  1-2  degrees 
of  latitude.  Within  these  bounda- 
ries, the  s\^  is  never  vertical ;  nor 
does  he  ever  remain,  during  a  whole 
day,  below  the  horizon. 

Thermometer.  An  instrument  for 
measuring  the  temperature  of  the 
atmosphere,  or  of  other  bodies. 

Torrid  Zone.  That  portion  of  the 
earth  which  extends  23  1-2  degrees 
on  each  side  of  the  equator,  to  the 
tropical  circles;  within  this  limit,  the 
sun  is  vertical,  twice  in  the  year. 

Transit.  Mercury  or  Venus,  are 
said  to  transit  the  sun,  when  they 
pass  between  the  earth  and  that 
luminary.  They  then  appear  like 
dark  spots,  upon  the  face  of  the 
sun. 

Transparent.    Allowing  the  rays 


of  light  to  pass  freely  through.  The 
reverse  of  opaque.  Glass,  water, 
air,  &c.  are  transparent  bodies. 

Tropics.  Two  circles  on  the  globe 
on  either  hemisphere,  at  the  dis- 
tance of  23  1-2  degrees  from  the 
equator.  Beyond  these  circles,  the 
sun  is  never  vertical :  and  the 
countries  within  them,  are  denomi- 
nated tropical. 

Twilight.  That  portion  of  the 
morning  or  evening,  in  wliich  the 
light  of  the  sun  is  perceptible,  al- 
though he  is  below  the  horizon. 

Vacuum.  Space  void  of  matter. 
Such  is  supposed  to  be  the  space  in 
which  the  planets  revolve.  We 
are  said  to  produce  a  vacuum, 
when  we  exhaust  the  air  from  a 
receiver. 

Valve,  A  part  of  a  pump,  and  of 
some  other  instruments,which  opens 
to  admit  the  passage  of  a  fluid  in 
one  direction,  but  closes  when 
pressed  in  the  opposite  direction, 
so  as  to  prevent  the  return  of  the 
fluid ;  a  pair  of  bellows  is  furnished 
with  a  valve. 

Vapour.  Exhalations  from  fluid  or 
solid  substances,  generally  mixing 
with  the  atmosphere.  The  most 
abundant,  is  that  from  water. 

Vertical.  Exactly  over  our 
heads :  ninety  degrees  above  our 
horizon. 

Vibration.  The  alternate  motion 
of  a  body,  forwards  and  backwards; 
swinging,  as  a  pendulum. 

Visual,  Belonging  to  vision;  tis 
the  visual  angle,  or  that  angle 
formed  by  the  rays  of  light  which 
enter  the  eye,  from  the  extremities 
of  any  object. 

Undulation.  A  vibratory,  or 
wave-like  motion  communicated 
to  fluids.  Sound,  is  said  to  be  pro- 
pagated by  the  undulatory,  or  vi- 
bratory motion  of  the  air. 

Weage.  One  of  the  mechanical 
powers  ;  the  form  of  the  wedge  is 
well  known.  It  is  of  extensive  use ; 
serving  to  rend  bodies  of  great 
strength,  and  to  raise  enormous 
weights. 

Wheel  and  Axle.  One  of  the 
mechanical  powers,  used  under  ya«. 


GLOSSARY. 


215 


riotts  modifications.  Cranes  for 
raising  weights,  the  wheels  and 
pinions  of  clocks  and  watches, 
windlasses,  &c.  are  all  applications 
of  this  power. 

Zodiac.  A  broad  belt  in  the  hea- 
vens, extending  nfcarly  eight  de- 
grees on  each  side  of  the  ecliptic ; 
the  planes  of  the  orbits  of  all  the 
planets  are  included  within  this 
space.  This  belt  is  divided  into 
twelve  parts  or  signs,  each  contain- 
ing 30  degrees. 

These  signs  are : 
Aries;  the  Ram. 
Taurus;  the  Bull. 
Gemini;  the  Twins. 
Cancer;  the  Crab. 
Leo;  the  Lion. 
Virgo  ;  the  Virgin. 
Libra;  the  Scales. 
Scorpio ;  the  Scorpion. 
Sagittarius ;  the  Archer. 
Capricornus ;  the  Goat. 
Aquarius;  the  Waterer. 
Pisces;  the  Fishes. 

The  first  six  are  called  northern  signs; 


because  the  sun  is  in  them,  during 
that  half  of  the  year,  in  which  he  is 

vertical  to  the  north  of  the  equator; 
tlie  last  six,  are  called  southern 
signs  ;  because,  during  his  journey 
among  them,  he  is  vertical  to  the 
south  of  the  equator. 

The  sun  enters  Aries,  at  the  time  of 
the  vernal  equinox ;  Cancer^  at  the 
summer  solstice ;  Libra,  at  the 
autumnal  equinox ;  and  Capricor- 
nus^ at  the  winter  solstice. 

The  sun  is  said  to  enter  a  sign,  when 
the  earth  in  going  round  in  its  orbit, 
enters  the  opposite  sign.  Thus, 
when  the  sun  appears  in  the  first 
degree  of  Libra,  it  is  in  conse- 
quence of  the  earth  having  arrived 
opposite  to  the  first  degree  of  Aries. 
A  line  then  drawn  from  the  earth, 
and  passing  through  the  centre  of 
the  sun,  would,  if  extended  to  the 
fixed  stars,  touch  the  first  degree 
of  Libra. 

Zone.  The  earth  is  divided  into 
zones,  or  belts.  See  Frigid,  Tem- 
perate, and  Torrid  Zones. 


INDEX. 


A. 
Air,  11,  15, 28,  50, 136. 
Air-pump,  31, 145. 
Angle,  44. 

acute,  44. 
obtuse,  44. 
right,  44. 

of  incidence,  45, 154,  160,173. 
of  reflecUon,  45,  154,  160, 173. 
visual,  168,  169,  170. 
Angular  velocity,  171. 
Antarctic  circle,  92. 
Aphelion,  75. 
Arctic  circle,  92. 

Atmosphere,  28,  104,  129,  136,  144, 
150, 163. 
colour  of,  193. 
reflection  of,  193. 
refraction  of,  182. 
Attraction,  10,  14,  23, 25,  179. 

of  cohesion,  15,  19,  118. 
capillary,  18. 

of  gravitation,  18,  23,  29, 
70,80,96,116,  136. 
Avenue,  170. 
Auditory  nerve,  151. 
Axis,  78. 

of  motion,  48. 
of  the  earth,  22,  99. 
of  mirrors,  176. 
of  a  lens,  184. 

B. 

Balloon,  30. 
Barometer,  140. 
Bass,  155. 
Bladder,  138. 
Bodies,  10. 

elastic,  40. 

fall  of,  23,  26,  30,  36-. 

luminous,  157. 

opaque,  157. 

sonorous,  152,  155. 

transparent,  157. 
Bulk,  16. 

C. 

Camera  obscura,  184,  197,  201. 


Capillary  tubes,  18. 
Centre,  48. 

of  gravity,  48,  51,  52,  115. 

of  magnitude,  48,  53. 

of  motion,  48,  55,  115. 
Centrifugal  force,  49,  72,  95, 115. 
Centripetal  force,  49,  72. 
Ceres,  84. 
Circle,  44,  94. 
Circumference,  94. 
Clouds,  129. 
Colours,  23,  185. 
Comets,  86. 
Compression,  42. 
Concord,  155. 
Constellation,  86. 
Convergent  rays,  175,  177. 
Crystals,  12. 

Cur\'ilinear  motion,  47,  72. 
Cylinder,  52. 

D. 
Day,  78,  105, 106. 
Degrees,  44,  94,  99,  169,  170. 
of  latitude,  94,112. 
of  longitude,  94,  112.. 
Density,  16. 
Diagonal,  47. 
Diameter,  94. 
Discords,  155. 
Diurnal,  78. 

Divergent  rays^  175,  177. 
Divisibility,  10,  12. 

E. 
Earth,  18,  70,  84,  88,  95. 
Echo,  154. 
Eclipse,  110,  159. 
Ecliptic,  86,  92,  99. 
Elasticity,  41. 
Elastic  bodies,  28, 40. 

fluids,  28,  41,  118,  136. 
Ellipsis,  75. 
Equinox,  100,  107. 

precession  of,  107, 
Equator,  92,  99. 
Essential  properties,  10, 
Exhalations,  13. 
Extension,  10,  11. 


218 


INDEX. 


Eye,  166,  195. 


Jupiter,  85. 


F. 


Fallofbodies,  24,  27,31. 
Figure,  10,  12. 
Fluids,  118,  128. 

elastic,  28,  41,  118,  136. 

equilibrium  of,  120, 122,  132. 

non-elastic,  119. 

pressure  of,  121. 
Flying,  40. 
JPocus,  176. 

of  concave  mirrors,  177. 

of  convex  mirrors,  175,  177. 

of  a  lens,  184. 

imaginary,  176. 

virtual,  176. 
Force,  33. 

centrifugal,  49,  72,  95,  115. 

centripetal,  49,  72. 

projectile,  47,  49. 

of  gravity,  47,  49 
Fountains,  135. 
Friction,  68,  69,  135. 
Frigid  zone,  93. 
Fulcrum,  54. 


General  properties  of  bodies,  10. 
Georgium  Sidu«,  85. 
Glass,  183. 

burning,  188. 

refraction  of,  183. 
Gold,  119,  126. 
Gravity,  18,  23, 78,  97. 

H, 

Harmony,  155. 
Heat,  16,  29, 103. 
Hemisphere,  92, 100. 
Herschel,  85. 
Hydraulics,  118. 
Hydrometer,  128. 
Hydrostatics,  118. 

I. 

Imaige  on  the  retina,  165,  172. 

reversecl,  167. 

in  plain  mirror,  172. 

in  concave  do.  175. 

in  convex    do.  175. 
Impenetrability,  10. 
Inclined  plane,  54,  66. 
Inertia,  10, 14,  32. 
Inherent  properties,  10,^ 
funo,  84. 


L. 


Lake,  133,  135. 
Latitude,  94,  112. 
Lens,  184. 

concave,  184. 
convex,  184. 
meniscus,  184. 
plano-concave,  184. 
plano-convex,  184. 
Lever,  54,  55. 

first  kind,  58. 
second  kind,  60. 
third  kind,  60. 
Light,  157. 

pencil  of,  158. 
ofthemoon,  162, 163. 
absorption  of,  188. 
reflected,  160. 
refraction  of,  179. 
Liquids,  118. 
Longitude,  94,112. 
Luminous  bodies,  157. 
Lunar  month,  108. 
edipse,  110. 

M. 

Machin©,  54, 66. 

Magic  lanthom,  203. 

Mars,  84. 

Matter,  10,  13. 

Mechanics,  32. 

Mediums,  137, 180. 

Melody,  156. 

Mercury,  (planet)  83,  85,  114. 

Mercury,  or  quicksilver,  16, 140, 141 

Meridians,  93. 

Microscope,  200. 

single,  200. 
double,  200. 
solar,  202,  203. 
Minerals,  12. 
Minutes,  94. 
Momentum,  38,  56. 
Monsoons,  149. 
Month,  lunar,  108. 
Moon,  78,  79,  80,  82,  85. 
Moonlight,  162,  163. 
^lotion,  14,  32,  36. 

accelerated,  36. 

axis  of,  48. 

centre  of,  48, 55. 

compound,  46. 

curvilinear,  47,  49. 

diurnal,  78. 


INfiEX. 


219 


Motion,  perpetual,  35. 
retarded,  35. 
reflected,  43. 
uniform,  34. 
Mirrors,  172. 

axis  of,  176. 

burning,  177. 

concave,  174,  176, 209. 

convex,  174,  175. 

plane  or  flat,  172. 

reflection  of,  173. 

N. 
Neap  tides,  1 16. 
Nerves,  166. 

auditory,  151,  166. 

olfactory,  166. 

opticv  164,  166. 
Night,  78. 
Nodes,  110. 

O. 

Octave,  156. 

Odour,  13. 

Opaque  bodies,  157,  158. 

Optics,  157. 

Orbit,  86. 


Pallas,  84. 
Parabola,  51, 
Parallel  lines,  25. 
Parallel  of  latitude,  94. 
Pellucid  bodies,  157. 
Pencil  of  rays,  158. 
Pendulum,  98. 
Perihelion,  75. 
Perpendicular  lines,  25. 
Phases,  109. 
Piston,  143, 145.       - 
Plane,  92,  93. 
Planets,  76,  81,  83. 
Poles,  92,  99,  100. 
Polar  star,  100,  112. 
Porosity,  42,  126. 
Powers,  mechanical,  54. 
Projection,  49,  50,-71. 
Precession  of  equinoxes,  107. 
Pulley,  54,  63. 
Pump,  31. 

sucking  and  lifting,  143. 

forcing,  144,  145. 

air,  31,  145. 
Pupil  of  the  ©ye,  164. 


Rain,  17,  129. 
Rainbow,  188. 
Rarity,  16. 
Ray  of  light,  158,  179. 

reflected,  160,  161. 
incident,  161. 
Rays,  intersecting,  165. 
Reaction,  39. 
Receiver,  31. 
Reflection  of  light,  160, 163. 

angle  of,  45,  161,  173. 
of  mirrors,  173. 
of  plane  mirrors,  174. 
of concave  do.  174. 
of  convex  do.  174. 
Reflected  motion,  43. 
Refraction,  179,  186. 

of  the  atmosphere,  182. 
of  glass,  183. 
of  a  lens,  184. 
of  a  prism,  185. 
Resistance,  54. 
Retina,  165. 

image  on,  166 
Rivers,  134. 
Rivulets,  131. 

S. 
Satellites,  80,  111,  113. 
Saturn,  85. 
Scales,  or  balance,  56. 
Screw,  54,  67. 
Shadow,  110,  111. 
Siderial  time,  106. 
Sight,  165. 

Signs  of  the  zodiac,  86,  93. 
Smoke,  14,  29. 
Solar  microscope,  202. 
Solstice,  100,  102. 
Sound,  151. 

acute,  155. 

musical,  155. 
Space,  33. 
Specific  gravity,  123. 

of  air,  140. 
Spectrum,  190. 
Speaking-trumpet,  154. 
Sphere,  26. 
Springs,  130. 
Spring  tides,  116. 
Square,  81,  85. 
Stars,  77,  8^,  102. 
Storms,  147. 


220 


INDEX. 


Substance,  10. 

Vibration,  98, 152. 

Summer,  76, 100. 

Vision,  164,  168. 

Sun,71,75,78,162, 

182. 

Vision,  angle  of,  168, 170. 

Swimming,  41. 

double,  171. 

Syphon,  132. 

U. 

T. 

Undulation,  153. 

Tangent,  49,  73. 

■Unison,  155. 

Telescope,  203,  204. 

reflecting, 

204. 

W. 

refracting 

,204. 

Water,  118,  130. 

Temperate  zone,  92, 

101. 

spring,  130. 

Thermometer,  142. 

rain,  130. 

Tides,  114,  116. 

level  of,  120. 

neap,  116. 

Wedge,  54,  66. 

spring,  1 16. 

Weight,  23. 

jerial,  150. 

Wheel  and  axle,  54,  65. 

Time,  105,  107. 

Wind,  146. 

siderial,  107. 

trade,  147. 

equal,  107. 

periodical,  148. 

solar,  107. 

Winter,  76,  101. 

Tone,  155. 

Torrid  zone,  93,  147. 

,182. 

Y. 

Transit,  114. 

Year,  107. 

Transparent  bodies, 

157. 

siderial,  107. 

Treble  and  bass,  155 

solar,  107. 

Tropics,  92. 

•y 

V. 

Zodiac,  86. 

Valve,  143. 

Zone,  93. 

Vapour,  17,  29, 104, 

129. 

.  torrid,  93,  147, 182. 

Velocity,  33,  67. 

temperate,  93, 101.  ' 

Venus,  84. 

frigid,  93, 100. 

Vesta,  84. 

THE    END. 


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